Crowley and Aziraphale need friends
Am I the only one who thinks that spending their future as a couple completely isolated from the other angels and demons wouldn't be the ideal ending for Crowley and Aziraphale? To be clear, there's nothing wrong with wanting to spend your life just with this one special person. But it makes a difference if this is a conscious choice or if it's literally your only option. For millenia, Crowley and Aziraphale had no one except each other. They are different from the other angels and demons, and that made them feel lonely, like they both admitted in the Job minisode. In the book, where their relationship is arguably less emotional than in the series, a big part of the reason why Crowley and Aziraphale hang out with each other is that "you grew accustomed to the only other face that had been around more or less consistently for six millenia" (Pratchett/Gaiman, p. 52f.; cited below).
Crowley and Aziraphale cling to each other because they love each other, but also because they have no alternative if they don't want to be completely on their own. And I don't think that this form of emotional dependency is a healthy basis for a romantic relationship in the long term. So in my opinion, they need friendships with others as well.
Fans have suggested the people from Whickber Street as potential friends for Crowley and Aziraphale. But if having friendships with humans were a solution to their problem, then they could have had that before. We know that both of them have interacted positively with humans throughout history, but I don't believe that real friendships with them are possible. Crowley and Aziraphale are immortal, and humans only have short lifespan, so, as Crowley puts it, "you can't let yourself get to attached to them". Moreover, there's a huge gap both in power and in knowledge between them. I mean, what common ground do a being who had a part in creating the universe and witnessed the evolution of mankind since the beginning and a regular human have?
Plus, Crowley and Aziraphale still regularly exploit said power gap. @ineffable-sideburns illustrated this masterfully in this post, quoting Granny Weatherwax from Terry Pratchett's Discworld series: "And sin, young man, is when you treat people like things". Treating people like things is definetely what heaven and hell do; they view them as nothing but pieces in their cosmic game of chess. But even Crowley and Aziraphale, being so vastly different from their fellows, have not yet completely moved beyond that way of thinking.
Our angel and demon still believe it's okay to manipulate humans to serve their own interests. They want a bus to drive to London instead of Oxford? They do a miracle so the driver takes a different direction, even if he "won't know why". They want to sit at a table that is occupied? They make the people who sit there get up. The most extreme example of this is probably Aziraphale's ball, were he makes everyone "talk like they've escaped from Pride and Prejudice" and Mrs Sandwich isn't able to pronounce the name of her profession, in order to fit the atmosphere (cf. the post cited above).
Crowley and Aziraphale even try to mess around with Nina's and Maggie's love life, utterly ignoring the fact that Nina already has a partner. Their main motivation is to cover up what their 25-lazarii miracle really was used for. And although they both state that they also want to help Nina and Maggie, that does not diminish the fact that the way they go about it completely disregards the two women's freedom of choice. Nina and Maggie directly address this issue: "We're not a game, we're real people! You cannot just pair us up for your amusement" (cf. the post cited above). In short, Crowley and Aziraphale prove they still regard humans partly as their toys. They mostly play nicely with them instead of destroying them like heaven and hell do, but they still do play with them.
And they might - hopefully - overcome that attitude, but it is one of the reasons why I don't think deep friendships with humans would work very well. So when I say they need friends, I think they should find them within their own species - and by species, I mean angels, fallen or not. We have already seen three possible candidates for a friendship with Crowley and Aziraphale, and I will go through them one after another.
The first and most obvious one is Muriel. They're kind and cheerful and they don't seem obsessed with heaven's ideology. They show no problem with interacting with Crowley and Aziraphale, even if the two of them are "the traitor and the demon", and are even interested in spending more time with them.
However, the problem is that neither Crowley nor Aziraphale take them seriously. The duo makes fun of "Inspector Constable" and their poor attempts to pass as human. Granted, Aziraphale is more supportive, prompting Muriel to give appropriate answers. But Crowley intentionally asks questions that he knows Muriel can't answer, like the one about which "human settlement" they come from, just for entertainment. There really is no reason for that, especially if you are the one who, after 6,000 years on earth, still thinks that humans fall in love by looking into each other's eyes. Yes, Muriel was sent as a spy, but that's not their fault and no excuse to make them the butt of the joke.
And in the last episode, when Muriel thinks that they will be included in Crowley's and Aziraphale's "us time", too, Crowley rids them of their illusions in a not very gentle way. It is understandable that he likes to go to the Ritz with Aziraphale alone, but he could have suggested that Muriel can accompany them in the future. Instead, he sends them off very quickly and makes clear that he does not want to further socialize with them, even though they just massively helped him in heaven (for a more thorough analysis of the relationship between Crowley and Muriel, I suggest reasing this post by @snek-eyes). To sum it up, Aziraphale does not get to spend much time with Muriel, and Crowley dismisses any possibility of a friendship with them right from the beginning.
Next up, we have Shax. To be honest, she would probably be more of a friend for Crowley, since she seems to find Aziraphale kind of ridiculous and does not understand what Crowley sees in him. However, she accepts that he means very much to the demon and is hesitant to hurt him even during the attack on the bookshop, using threats and insults and giving him multiple chances to surrender Gabriel rather than going full violent.
I've already mentioned in my post about Shax and FurFur that for someone who has been stuck in hell's toxic environment for millenia, she appears to be a pretty decent person. She even adopts Crowley's strategy of presenting herself in front of hell far more evil than she actually is. And I've yet explored her relationship with Crowley a bit, how she seems to like him, offers him a deal to their mutual benefit and warns him that he and Aziraphale will be in big trouble if they are found hiding Gabriel.
And Crowley is not opposed to hanging out with Shax, either. He teaches her how to properly navigate life on earth and answers her questions (well, we know that Crowley does not like it when people get upset about questions anyway). He even opens up to her a little bit about his feelings. Certainly, he is not fond of Shax threatening Aziraphale and himself. But he is probably also aware that her job requires her to do these things, because he's been living under hell's rule himself. And yet, he seems to look down on Shax for being incompetent, calling her "an idiot" and "stupid" in front of Aziraphale and others and making fun of her every time she does not get whether he is being sarcastic to her or not. Crowley is more willing to interact with Shax than with Muriel, probably because they're both demons and share the experience of hell's cruel regime, but he doesn't take her serious enough or trusts her enough to truly consider her a friend.
The third and last candidate for a friendship with Crowley and Aziraphale is Saraqael. Her motives are still unclear, but there have been hints that she might be a possible ally for the two of them in the finale (for more about Saraqael, see this post). Once again, it is Crowley who gets to interact more with her; when she visits Aziraphale, she is accompanied by the other Archangels and behaves accordingly, without any display of solidarity. But in her conversation with Crowley, she reminds him of how they used to work together. Yet, Crowley brushes her attempts to build a common ground aside. To be clear, I believe that Crowley truly doesn't remember her, but when she clarified who she was, he could have simply said "Sorry, I didn't recognize you" or something alike. Instead, he makes clear that she is simply not important enough for him to remember her. "I meet a lot of people" said with a laugh is, in my opinion, definetely meant as an insult, and is perceived by Saraqael as such.
So, in conclusion, there are several (fallen) angels who could become friends with Crowley and Aziraphale, and Aziraphale would probably not be opposed to it. After all, he is generally more open towards people and was willing to give even someone like Gabriel a second chance. However, Crowley turns down everyone who tries to get close to him immediately. He is not overly hostile to them, but he regularly mocks them to maintain the distance between him and them. Initially, he even tried to do the same with Aziraphale. In Eden, Crowley attempted to make fun of him for losing his sword. It was only Aziraphale's confession that he gave it away that completely changed Crowley's perception of the angel and encouraged him to get to know him better.
A big part of that attitude probably stems from bad experiences in the past. As Nina says, he's "the hard-bitten one, the 'can't trust anybody ever again'". But another reason is that he considers himself and Aziraphale as somewhat superior to the other angels and demons. In the Final Fifteen, when Aziraphale suggests going back to heaven, Crowley replies "we're better than that". Heaven and hell might be fit for others, but not for them. In my opinion, that's a very arrogant mindset, and also one that fails to acknowledge the fact that Crowley and Aziraphale are quite privileged compared to their fellows. Yes, they have always been acting more moral than all the other angels and demons, but the circumstances for them have also been different.
While they were both still technically under the surveillance of their respective superiors, they got to spend the biggest parts of their lives far away from them, enjoying earthly pleasures and doing mostly what they liked. Compare that to the reality of the demon Josh, for example, who says: "Every day that I don't have my tongue ripped out is a good day". People like him could not afford to disobey orders even if they wanted to, they would be punished instantly. So Crowley should not act as if all the other angels and demons were fundamentally different from them. It's a difference in experience and living conditions rather than in nature.
If Crowley manages to stop regarding everyone as their enemy and give people a chance, he (and by extension, Aziraphale) might be able to form new bonds and maybe even gain new allies for what's to come, and that will have a positive impact on their own relationship, too.
Pratchett, Terry and Gaiman, Neil: Good Omens. The Nice and Accurate Prophecies of Agnes Nutter, Witch. London 2011.