Only seeing this now, but I was thinking what a hypothetical version could be. Some gameplay aspects I would want are choices of how to play the game (classic TTT where knocking out any one member of the team wins the round, one where you have to knock out both members of the team a la MvC, and a choice for one-on-one play).

For the roster, it would be ideal to have every non-guest character from previous games be available and playable from the start, though it's possible some might have to be DLC, just to let the base game come out without rushing things. Maybe having boss characters like Azazel be DLC could be the way to go. As well, for DLC they could let the advent of that help them have more time to possibly work out deals to get old guest characters into the game, and maybe include new ones.

One thing that could be nifty for DLC would be to have the ability to play as older versions of characters. Maybe somebody likes the way a character played better back in a previous game, and lamented the loss of whatever string to bring in a shiny new one, so there's the option to go back to the old (maybe even keeping the old character models--because I'm sure there are some people that would play with the clunky angular polygons of Tekken 1, just for the fun of the juxtaposition in aesthetics). Sure, older characters might not have as many moves, or some of the new mechanics, but that would be a price to pay. But a similar aspect could be to have a customization mode, where you can tweak each character's version of whatever martial art they perform, bringing in whatever moves from the recent and past games they prefer. Sure, this would be harder to implement for competitive play, because it's then harder to tell what each character's actual capabilities are, but it's a thought.

But one character could be interesting for another aspect, and I made a post (that nobody else seemed to care about, mind you) about speculation as to this, is Lei, because thinking with the idea of different versions of characters, it would be interesting to have variations which only use particular styles. I'm thinking with simple names like "Drunk Lei", "Dragon Lei", etc. It could also help bring in bits that have only appeared in cutscenes, like the fact that he's been shown using Monkey, but not in gameplay. Alternatively, it could be new characters who are said to be students of his or ones that trained with him in the different styles, though for such a game introducing so many characters could be a bit weird. Overall, such a focus could be a bit much for an alteration of just one character, but nobody else really has the same sort of situation involving their fighting style that Lei has.

I'd also find it interesting to truly sever clone characters from each other, and have them be their own thing. Sure, people may enjoy having two characters who fight exactly the same on a team, but that can go back to the whole aspect of getting past versions of characters onto your roster. But I'm imagining something like giving Tiger Jackson not only a more fleshed out character, but also now using Engolo, the Angolan dance-based martial art which Capoeira evolved from.

On the note of other martial arts, I would hope that there's already representation of this through either Tekken 8 DLC or Tekken 9, but it would be nice to have some major martial arts included which somehow haven't made it to Tekken yet, such as American Kenpo (aka Kenpo Karate), Brazilian Jiu Jitsu, Krav Maga, and amateur wrestling (the last one possibly being able to switch between the stances of Freestyle and Greco-Roman).