RENDERS!
Pretty sure these are wii-u version, some of the previous ones likely too
I wouldn't limit this by gen, rather by what characters are going to make the most unique roster for the first game. Sequel can focus a bit more on gens. Pokemon has never been about gen representation, rather that each one adds something to the legacy while keeping fan favs alive. Steelix is kinda ruled out by the fact the game is doing canon scaling, which means he's be what, the size of the arena? I think/hope we're reaching the end of the starters, I'd say that Blastoise, and maybe Greninja are likely, since the former would be siiccckkkk, but the latter is kinda teetering since it'd make good sequel bait.
Now back on Snake types, well we got Arbok, Dunspace, Seviper, and Superior (and of course the first and third have their first evolutions, but they're both kinda lame). Of the four, well Dunspace does nothing, and Arbok is essentially a classic Cobra, pretty cool, but not very complicated. I'd say Seviper would have the most interesting move-set, since it's got general serpent style moves, but also could be a wierd sorta sword fighter. Now on another forum had a fellow member suggest Gyrados, Milotic, Huntail, Gorbyss, or Eelektross, but half of those are too big(as in arena size) 2 are, kinda bland, and well Elektross has good move potential, but has Limbs which kinda beats the point. So yeah, putting my bets on Seviper.
Also, I still think Blastoise is the most likely Water starter, he's still the most iconic