Generations 2, a bold idea.
If it were ever to happen in the current time frame, it'd be based off the same physics engine that Lost World - Shadow Gens is based off of, which in my opinion needs to be scrapped for a more robust engine.
It'd probably be split up into 4 eras if Sonic Team were to entirely reuse the Generations concept, and by that, I mean not innovate. By the time it's come out, the Sonic Frontiers-and-onwards era probably has a few games on its belt, leaving at least 3 interesting places to choose from.
But rather than doing Generations again, I WOULD like to try to innovate. Specifically, in the control and in the stages.
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For some reason, people didn't like Shadow Gens' hub. I quite liked it! The next Sonic Generations would probably do something similar, having multiple stages scattered around a large hub, not split up into specific eras, allowing more freedom in choosing stages.
As for the stages themselves, it'd probably go something akin to the list below. I'm splitting them into eras for simplicity's sake. I also won't cover bosses because I don't feel like it, lol
Classic Sonic:
Starlight Zone
Mystic Cave Zone
Flying Battery Zone
Metallic Madness Zone
Adventure Sonic:
Emerald Coast
Metal Harbor
Rail Canyon
Kingdom Valley
Boost Sonic:
Dragon Road
Jungle Joyride
Asteroid Coaster
Sunset Heights
Open Sonic:
Chaos Island
???
???
???
Now, you can see there are quite a few of Shadow Gens picks. They're just great picks, and there aren't too many good alternatives. Also, 2 Unleashed stages because there were so few Boost games, and the one mostly considered the best, the one we're talking about making a sequel to right now, reuses stages.
As for ???, Open Sonic needs more games for more picks, so that'll have to wait.
Okay, controls. First and foremost, who are the playable characters? Call me crazy, but we're ditching Classic Sonic. I'd rather do something new, or rather OLD with current Sonic.
Let me quickly lay out some controls, and I can comb over them afterwards
A - Jump, Double Jump
B - Spindash, Dropdash
X - Boost, Airdash, Homing Attack
Y - Slide, Stomp
Okay, we're gonna try to merge Boost and Adventure!
First of all, the Boost has been nerfed like Frontiers - it does no damage. It's also a bit slower, not blasting you off in the same way. It's not intended to be held for the entire stage now.
Second off, the Spindash does in fact behave like in Adventure. It doesn't fit under gaps, though.
Thirdly, the Slide still serves the purpose of quickly sliding under gaps, and we need the Stomp to maintain speed.
Alright, rather than going over specific physics details, you can try to imagine this combination in your head.
Anyways that'd be a bare-bones pitch for Gens 2. Its main purpose would be to bridge the gap between Adventure (myself) and Boost fans more.