All right, you asked me a question and now I have one for you, in a very similar vein: where do you see Matthew Fairchild post-canon? What are your hopes for him? If you were the writer, what would you do?
Sorry this took me a couple of days to respond to! I'm here(ish) now.
I actually am extremely happy with the direction Matthew's story has taken in the canon thus far. Even if I was dissatisfied with some other arcs/writing choices (see: Grace's, Christopher's, Cordelia's in many ways), Matthew's storyline was handled in a sensitive, realistic, and character-driven way that I loved. And, of course, I'm excited to see him going forward. I think that Cassie is going to continue to do an amazing job with his story because she has not dropped the ball on him yet.
This is probably a cop-out, and I'll give more of an answer in a minute, but I just want Matthew to be happy and healthy. That's my only real hope for him in the future. Think that song My Wish : "My wish for you is that this life becomes all that you want it to / your dreams stay big / your worries stay small", yadda yadda. But that's my real answer. I just want a happy Matthew and don't care what form that takes. I am going to therefore be unequivocally satisfied with any story direction in his bindup since Cassie already basically said at a signing that he would get a happy ending.
But if I were Cassie, here's what I would do.
I'm excited that Matthew has chosen to travel, because it's something that's also important to me. It's yet another way that Matthew's values and life goals mirror mine. I think that travel is freedom, and I am happy that Matthew is going to experience that freedom.
I think that in the bindup, I would take him to visit a ton of places all across the world. I don't want him to confine himself to Europe. I'd like to see him in South America, Asia, Africa - everywhere! I think this seems likely to happen, and am glad of it.
However, within the confines of "Matthew gets a partner abroad," I would like to see him not being with said partner 100% of the time. A big part of Matthew's travel should be finding himself. I'd like something where he travels alone most of the time, sees his love interest like once a month or so, and spends a couple of days with them. This would be the short story we see, and they would split up again until the next month, which would be the next story. This would give him ample time for solo travel, meeting interesting people, and taking the reins on his own adventure.
In my ideal world in canon, I probably would not have him meet his partner while traveling. I think he should meet someone in about 5 years, so he can get a grip on his newfound sobriety and freedom. Just, like from a realistic perspective. I think this would be the best thing for him in a realistic scenario.
HOWEVER, I am all for him getting a love interest now since it's something I'm really excited to see in the bindup. As a reader, I'm 1000% here for Matthew's adventure romance whirlwind. And I know that it will be totally fine in Cassie-Clare-Bookland, even if it's not what I think would be best for Matthew in a real life case.
For Matthew's love interest, I actually actively want him to end up with a woman. It's important to me as a bisexual to see a bi person wind up with someone of a different gender for once, and unlike a lot of people, I really trust Cassie to deliver with it. She hasn't disappointed me regarding Matthew before, and I don't see why she'd start now. I also would like to see an absolutely chaotic scapegrace of a girl who rejects Edwardian social conventions and embraces the bohemian lifestyle alongside Matthew.
Regardless of love interest gender, I want the person to be kind of a batshit bonkerballs sort that matches Matthew's chaotic energy. I want them to be comfortable in their freedom and their chaos; I want them to have scandalous fun together. I think Matthew needs someone who loves and embraces the crazier sides of himself, the mercurial moods, the fact that "feelings flow from him like blood from a cut." The fact that he's a little impulsive. This is what I deeply want for Matthew, and it's another reason that I'm chill with the idea of him meeting someone on his travels. It'll be another adventurer.
In my ideal world, Matthew would not have children. This is wish fulfillment because I am childfree. But I know he is going to have them and be Clary's ancestor. Though I'm not enthralled about that, I still don't think it will be harmful to his character. I think that he and the love interest will continue to be unabashedly themselves and raise their kids in a rabble-rousing bohemian way. But, if I'm taking creative license, I ain't giving him kids. I'm making one of the twins Clary's ancestor. I don't give a fuck about bloodlines, sorry.
I actually do have a controversial take regarding Matthew. If I were given full creative license over the character, I would give him a lot of distance from James and Cordelia. I hope that he goes no-contact with Cordelia while traveling so that he has time to heal from her and get over her, and I hope that he keeps his communication with James to periodic letters. I know he'll be close with them in the future, which is fine.
But if it were up to me, he wouldn't be. He'd still talk to them, be James's parabatai, but the emotional proximity would decrease.
I always viewed TMT very much as James/Matthew + Thomas/Christopher, but I'd want to bend the shape of this. I think James should become closer with Jesse, and Matthew should become closer with Thomas and Alastair. Not just because I like Thomastair better than others, but because I think that James and Matthew have so much hurt between them. And because, I cannot stress this enough, Matthew needs distance from Cordelia.
I think that Thomas's disposition complements Matthew's well, and I think that he could provide him and the chaotic love interest with a calming presence. I also think that Matthew and Alastair should become really, really close because they can understand each other in ways that other people cannot. Adding Grace to this BFF mix would also make me really happy, though I understand that it's unlikely.
That's where I currently am!