Well, I would prefer if they tried a new gameplay formula as I’m sure we all would, but if they had to:
Really show what the Hedgehog Engine 2 can do, we saw what Sonic Unleashed looked like 12 years ago and to this day that still holds up as visually one of the best looking Sonic games. Forces had the potential but it was rushed and lazy. I would want them to really push Sonic to the boundaries, not an anniversary game celebrating nostalgia other then a few call backs here and there. Remove 2D completely from the gameplay. (Colour’s biggest problem was that although you had a good act 1 and 2 usually. The other acts consisted of blocky, 2D sections with no momentum. Not really Sonic returning to its roots is it?) There isn’t really any need for 2D. Also drift would need to be brought back for obvious reasons.
Along with the visual ideas. Show us new landscapes. Take us to new levels, bring back music that captures the atmosphere too. (Apotos, Empire City)
Generations had the better level design for Modern (if I recall correctly) but looked slightly not as appealing as Unleashed. Have both of these things are we are golden, and we know this current generation and next generation can handle that.
Now, I would want the fine line between a light hearted story and a serious story. If it’s light hearted at least make sure the characters ACT like themselves. Sonic can still show attitude in a light hearted game. and it is different then comedy. I could rant about Graff and Pontac but I’m sure we’ve all heard it all before. Just focus on that “attitude” feel for Sonic. He’s meant to be cool. And I feel the games lost that leading me on to my next point:
Finally, a somewhat new art style or at least a new model for Sonic. We’ve had the same Sonic model since 2008 more or less (changing for the worst when Forces came out with that hideous washed out blue.) and while it looked great in Unleashed and Generations I feel it’s time for a switch up, Riders, Heroes and 06 all look visually distinct from each other and I would like to see a game tackle that again. This one seems like it’s a little far fetched but if so, give us a new style of cutscene we least because characters standing and talking half the time is boring. Give us some Jet Set Radio/ Sonic Riders art style cutscenes and openings.
Just realised I more or less got on a rant about my ideal Sonic game. But uh, yeah.
TL/DR - Make Sonic cool again and make the game immersive.