OK, but I partially disagree with this headcanon, and here’s why:
1) Bruce is totally playing Gamora. You don’t think Bruce Banner has played Dungeons & Dragons before? Bruce Banner has absolutely played Dungeons & Dragons before. He played all through high school and college and when Bucky announces the campaign Bruce jumps at the opportunity because he just misses it so much (mostly rose-tinted nostalgia goggles but). So he sits Bucky down and asks him for every bit of info he can on the setting and spends a whole night with a pot of tea drafting up the five-page backstory for his space assassin and her family tree and her struggle with her relationship with the villain and comes to Bucky with a fully-ready character sheet and a list of things Bucky will need to OK before Gamora hops in.
Bucky quietly resolves to integrate as much as he can into the story, mainly because Bruce came up with some better ideas than he’d had.
2) Tony is definitely playing Quill, because Tony has never played D&D before. You don’t get to be where Tony Stark is in life and have much free time. He does what a lot of newbies do and bases a character on himself, or at least the parts he likes: clever, snarky, pre-’90s musical taste, beds space babes, heroic sometimes probably. He wants to be cool but has no idea how to be cool within this context (“My character’s name is Starlord.” “What? Tony, no.”). He hogs the spotlight all the time (all the time) but clearly has no idea what he’s doing and when someone who seems like they know what they’re talking about gives him advice he always takes (“I’m going to need that guy’s leg.” “Seriously? Alright” *Rolls to grapple*).
Quill’s backstory is primarily Bruce’s doing. Tony just handed it in with a “yeah whatever’s on there.”
3) Thor is playing Drax but didn’t join until a few sessions in when he tagged along and decided it looked like fun (“THIS PLEASES ME! ALLOW ME TO JOIN YOUR TALES OF ADVENTURE!”). He definitely needed help constructing his character sheet, but he had no problem coming up with a character. Bucky asked him what he wanted to play and got that glint in his eye and responded “I WILL FORGE A HERO WORTHY OF THE ANCIENT TALES OF ASGARD.” And he put a lot of thought into Drax, both in personal history and personality. He’s mostly modeled on Thor’s favorite Asgardian folk heroes, with some personal flaws and quirks thrown in that Thor thinks are interesting.
Of course Thor doesn’t really understand the game part of it, he’s in it for the story (“Thor what the hell man there’s no way we can take on Ronan at this level!” “AH BUT THINK OF THE THRILLING DRAMA OF THE MOMENT DRAX AND RONAN MEET AGAIN!” “We are all going to die.” “AND IT WILL BE A THRILLING TRAGEDY!”)
4) Steve is absolutely playing Rocket but what started as a complete joke ballooned into a fully fleshed-out character with a tragic backstory. Steve’s an artist, he’s a creative guy and little too creative for his own good sometimes and bouncing his ideas off of Natasha turned a simple joke into a more elaborate character dynamic than even Bruce’s. He trolls Bucky a lot and it’s even better for Steve when he really gets into Rocket’s character and plays up the drama, partly because Bucky can’t tell if he’s joking or not.
5) Somewhere in the brainstorming session, Steve and Natasha decided that Rocket has a partner who is a talking tree. Natasha pitches this idea completely straight-faced to Bucky and after the fiasco of Steve’s character idea Bucky’s just too tired to say no to the tree-man. Natasha gives him a bit of a backstory and how Rocket and Groot got together and it sounds pretty solid, so whatever, tree-man can stay.
Then when all the characters get introduced Natasha just hovers over Tony and puffs out her chest and says in her deepest voice: “I am Groot.”
And Steve snickers and nobody has any idea why.
A session later Natasha is responding to everything Tony says with that same deep “I am Groot.” and Steve goes blue in the face trying to hold in his laughter and Tony cracks and the game has to pause for 10 minutes while Nat and Steve recompose themselves.
Nat also has a better grasp of the rules than Bucky realized and completely tweaked her character into being able to do basically anything she can justify. And it’s all right there in the book, Bucky can’t even argue from a rules standpoint. They’re only level 5 Groot shouldn’t be essentially bulletproof but through some loophole in the rules, yep, there it is.
Natasha Romanoff is trained to exploit weaknesses. Of course she’s a total munchkin.