This game should obviously be made to celebrate Sonic's 25th Anniversary. It should be made sometime after Sonic Team and Sega get new employees that will help them make new console Sonic games. It should also be made in to work after Sonic Runners releases and if it sells well. This game should be for the Wii U, PS4, Xbox One, Mobile Devices, and PC.
I'm not all in for a story, I'm just writing about a cast of characters and the most important aspect of a game, the gameplay.
I'm starting with the cast of characters, there should be 3 Sonics this time, I'm talking about, the Classic Sonic, the Modern Sonic, and the Sonic Boom Sonic. Modern Metal Sonic, Classic Knuckles and Classic Amy should appear in this game. But me, personally, I want more playable characters around. But, not to many. I mean, besides Sonic, I personally, would want to play as Tails and Knuckles. With the Classic Tails and Knuckles, the Modern Tails and Knuckles, and the Sonic Boom Tails and Knuckles. Sonic Boom Amy, should be a side character. Which reminds me, every one of Sonic's friends from Sonic Generations (including Shadow and Silver) should return in this game and play bigger roles. (But obviously, the Classic, Modern, and Sonic Boom versions of Sonic, Tails, and Knuckles should be playable. making them the main characters). Classic Eggman, Modern Eggman, and the Sonic Boom Eggman should team up and be the main antagonists. It should show the Sonic Boom versions of the characters (Sonic, Tails, Knuckles, Amy, Eggman, and also Sticks) what the Chaos/Super Emeralds are, and the Sonic Boom characters should show to the Classic/Modern versions of the characters what the Chaos Crystals are. The plot should be better; no Time Eater; it should show how each different version of the character meets each other/how they get there. Also, Classic Sonic should have a voice (Jaleel White)
Now for gameplay, Classic Sonic, Tails, and Knuckles should play as they did in Sonic 3 & Knuckles, while Modern Sonic plays how he did in Sonic Unleashed, Sonic Colors, and the first Sonic Generations. Tails and Knuckles should play similar to their Classic selves, but in 3D (with some 2D portions). The Sonic Boom versions of Sonic, Tails, and Knuckles should play similar to how they play in the infamous Sonic Boom: Rise of Lyric. But this time with no glitches, and Sonic can run faster. You should he able to switch between your characters. Like when play through the Classic versions of the Stages as the Classic Sonic, you can choose to be as Sonic solo or have Classic Tails follow you around like in Sonic the Hedgehog 2 and Sonic the Hedgehog 3 & Knuckles, but actually have better A.I. by helping you defeat enenies and like Sonic the Hedgehog 4: Episode II, he should be able to fly you up to places, you should also have the option to have Classic Knuckles follow you to, and be useful, also actually punch things, like in Sonic 3 & Knuckles, when you see him for the second time in Launch Base Zone, he puches you back a bit, he should punch again in this game. In the Modern stages and Boom stages, the same thing. But with the Modern stages, with no doubtm it should play similar to Sonic Heroes, but better.
You should get the option to play as the characters' super/hyper forms. Meaning you should be able to play as Super Sonic, Super Tails, Super Knuckles, Hyper Sonic, Hyper Knuckles, and for the first time, Hyper Tails. in any level you want. In all Classic, Modern, and Boom versions. They shouldn't change the music when you turn Super/Hyper like the first Generations. As the final boss, you should face off against the 3 Eggmen as the 3 Super/Hyper Sonics (your choice), and a big ol' mech. In bosses, you should be able to switch between which kind of Sonic, Tails, and Knuckles' you want and they should be together on the same screen by option, like if you want Classic Sonic, Modern Sonic, and Sonic Boom Sonic, or Classic Knuckles, Modern Knuckles, or Sonic Boom Knuckles, or Modern Sonic, Modern Tails, and Sonic Boom Knuckles, .O.K. it should also happen in a, expansive hub world, filled with Sonic's friends to talk to and should have the same options from the first Generations, you should even turn super/hyper in the bosses. but if you did that as Sonic Boom Knuckles, then he will need no help finishing off the boss.
There should be more music in this game, like Sonic Generations had 3 songs each from various Sonic games. But this time, it should have up to 15, and include tracks that missed out from the first game including The Doomsday Zone, Unknown from M.E. (Sonic Adventure version), and even the tune of fighting Eggman from the first original three Sonic games.
This game should be more dark than the first one, make the game make sense, make it similar, but not to similar to Sonic Adventure in terms of story, because it's overall story was cool (Except from what I've seen, Big's stages).
Anywaym that's all my ideas, if you more/want to state your opinions, post 'em below.