I might've said this in here before but honestly? Sonic Adventure 1 and 2 are known quantities and at this point I'd take an attempt at that over whatever random thing they'll think of next. Even if it's not as good as the original I'd still take Adventure 1/2 Again But Not as Good This Time over stuff like Forces and Lost World that are almost conceptually antithetical on the deepest level to what I think a good Sonic game looks like lol.
Bold Statement: And I say this as a sonic fan, not to suggest game design is easy, but due to advancements to how games are designed in general , them attempting sonic adventure 1 and 2 again on a baseline would probably be better than the OG 1 and 2. There would be less control issues, the knuckles type stages would go from " an annoyance to some " to totally fine. And just due to how consuming game deisgn is nowadays there is no incentive to include characters like big. He would just be an easter egg in the background and everyone would generally be satisfied with decision like that.
Now it isn't cut and dry, like I mentioned on a previous page the shooting gallery type stages in both of those games would have to be redone from the ground up. Just as game design has evovled some of the issues of the other two playstyles, it has evovled in such a way that reveals these sections to be rather boring nothing. You would have to redo the entire purpose of these levels. Should they be merely get to the end? Should they be more combat focused, should mechs/ Mecha have more utility in how it navigates the levels. Or even if you wanna have those levels at all.
Then there are the sonic levels where, i'm not saying they have been negated to nothing, but the people making these games haven't done anything like that since... sonic adventure 1 and 2. Even the levels put into generations are modernized versions of those levels and are nothing like how the originals were. Which kind of have exploration in them.
Thats why I would suggest they remake sonic adventure 1 and 2 . Get them used to designing levels and gameplay like that. So they can eventually move on to whatever is next where they use whatever philosphy were in those levels to expand on the premise.
So I wont say they are known quantities because even I think need to like remake 1 and 2 to kinda practice. But a modern touch on a lot of the issues those games had, fixes itself.