What is the relationship between Fireheart and Sandstar like in your AU?
Ah man you've opened the floodgates. These two take up so much space in my head. As with everything these are subject to change but this is what I've been thinking thus far:
They're actually quite silly with each other, though you wouldn't know that if you didn't live in Thunderclan. Fireheart is especially playful and Sandstar sometimes gets roped into his shenanigans. This is mostly in TP3 where Thunderclan is having something of a golden age and life is really good. They need it after TPB and TNP.
I called them mates and everyone else thinks of them as that, but I don't think they quite consider each other that. I'm leaning queerplatonic for them. They're close in a way that they've been through so much together and managed to come out of it stronger together. They adopted and raised two kits (although Sandstar was very "dad and the dog he said he didn't want" about it lol) and end up being each other's closest companions and confidants as time goes on.
Leader-Deputy Relationship:
Sandstar is a highly competent leader who does whatever it takes to make sure her clan is in top physical condition. Fireheart is highly influential and the heart (ha) of the clan. Sandstar is more utilitarian and practical while Fireheart is more charismatic and emotionally intelligent. They like to present a united front.
Neither of them came out unscathed by all the shit that happened in TPB. Fireheart doesn't ever lose his sense of justice but he is faster to lose his patience if he senses any possible hint of injustice or tyranny. Sandstar came out more open-minded about cats outside of the clans (gotta rewrite Bloodclan at some point... hmm) but she can be quick-tempered and bloodthirsty.
They also tend to disagree with how to go about things as Fire's more avoidant of bloodshed. They end up compromising more often than not if they both feel strongly about something. Sandstar is leader and her word is law. But she's known Fireheart for far too long to believe that'll stop him from going against her if he feels it's right.
Though they're less silly, they're still outwardly affectionate towards each other even in front of other clans. More verbally than physically though. Also if they both think you're being stupid you're about to get one of the worst tag-teamed verbal smackdowns of your life. Unless you're fully confident in what you're suggesting it's wiser to go to one OR the other as they're likely to be a little nicer about it. Well, Fireheart might be at least.
A lot of their dynamic as leader and deputy actually formed before they were put in official leadership positions. I'm giving Sandstorm a larger role in TPB (she suffers so badly from main character's love interest syndrome it's unreal). And I'm making a pretty big change in Rising Storm, namely that Bluestar refused to name another deputy after she exiled Tigerclaw.
Obviously the clan is incredibly worried by this. Thunderclan has gone from functioning like a well-oiled machine to having a leader in shambles and no deputy.
What ensues is the senior warriors taking over and Fireheart and Sandstorm sort-of accidentally becoming top contenders for possible deputies. I am aging the senior warriors up. What is really needed by Thunderclan is a strong, hard-working leader with a lot of vigor. Someone old enough to have some sense but young enough to be leading the clan into what could be many battles in the near future. The senior warriors on the other paw are close to retirement age.
Whitestorm and a couple other senior warriors try to encourage this and the two grow in popularity and closeness within the clan. When Bluestar eventually dies Thunderclan has a succession crisis for all of two seconds before Fireheart refuses the offer and Sandstorm takes the helm.
They compete over who is the best hunter in Thunderclan. Sandstar has natural talent. Fireheart has the experience of being sent out to hunt every time his energy got too high for Bluestar's liking. Sandstar argues it's her by sheer number. Fireheart argues it's him as even though he catches fewer, his outweigh hers. It's all in good fun.
They'll deny it but these two are such GOSSIPS. Blame Fireheart and his big ass ears. He brought Sandstar into the dark side with his inability to keep his nose out of everyone's business.
Fireheart will (subtly) encourage their children (Squilf) to break the code if he thinks the code is being stupid (canon TNP Firestar can fight me in an alleyway cause he would not fucking saayyy thaaaat). Sandstar would like him to stop doing that. He will not.
There is a nonzero chance that they knew Leafpool was pregnant and decided to not say anything about it.
They share the leader's den in winter (it's a lot bigger inside than cats think) and when it's rainy but usually Fireheart sleeps outside. He can technically fit in the warrior's den, but it's tight and cats are tired of being at the receiving end of a kick if he has a prophetic dream or something.