Backtagging: Yep! With texting tags, I'm comfortable with dropping after a bit. With planned out lines, I'm open to continue for as long as you're interested.
Fourthwalling: Most of the main cast in Honkai: Star Rail break the fourth wall at one point or another. It's usually done pretty casually/cheekily, but let me know if you'd prefer that I refrain from meta comments like this altogether.
Fourthwalling, 2.2 Spoiler: Additionally, Gallagher knows that he's a fictional meme created by Fictionologists with a single mission that needs fufilling. It can... weird to address. Gallagher knowing that he's not real is, honestly, a bit of a mindfuck, so I don't plan on bringing it up in threads unless everyone's on board with exploring the nature of reality.
Shipping: Yes!
Smut: Yes!

Pretty open to everyone at the moment! I'd love to see him interact with members of the Astral Express, and the people of Penacony, and characters from previous worlds.

Fave: Sunday, Caelus, Stelle
Yes: Aventurine, Robin, crosscanon, most characters in the Star Rail universe, tbh.
Maybe: Dr. Ratio, Mikhail
No: Kid characters, or kid-coded characters (Clara, Hook, Bailu, etc), medium/small Genshin Impact characters, cross-medium

Full kinklist, but these are Gallagher-specific things that I'd love to play out:

Misha: I think seeing Gallagher acting as like a weird stepdad to Misha could be really sweet. All things considered, it's pretty tragic, but I like the idea of Gallagher being stuck in this cycle of being loyal to Mikhail per his mission, and since Misha is basically Mikhail, looks like Gallagher is eternally on Dad Duty.