First and foremost they should keep going with Mania style games with the same dev team, perhaps while making them gradually more graphically complex as if they are releasing them on gradually more complex retro consoles. I'd happily take one every 2 years or so.
As for the 3D games: I'd say, go back to the drawing board after Forces and completely reboot them. They should look closely at
ShayMay's third Sonic Spitball video and create a new momentum based movement system where Sonic is no longer limited to going fast in semi-automated on-rails sections but instead can now freely navigate the environment and use it to gain speed and momentum (akin to what you'd see in a skateboarding game like Tony Hawk or even Jet Set Radio).
People often say Sonic was never meant to work in 3D but I think he would be perfect for 3D if they double down on his movement options and his interaction with the environment instead of on characters, spectacular set pieces and on-rails fast sections that offer little to no control depth (imo).
Oh, and no more humans beyond Eggman in either 2D or 3D Sonic games please. Give me natural landscapes, mysterious ruins, dazzling casinos and cities and ominous industrial zones but keep it unclear if humans live there or not, just like in the old games.
Release "Sonic Maker" with the assets from the Mega Drive games so that people who want more Mania games can make their own.
Could even put in a "procedurally generate random stage" button that would do the work for you.
Yes, I'd like to see this happen as well now that you mention it. Just let me create levels in all those great looking pixellated landcapes. Ideally this would include Sonic CD as well and an option to add time travel to switch between zones without exiting one.
