Why not both? 2D Classic games and 3D Modern games?
If Forces didn't have the 2 Sonics again it would have hit better with fans. Generations did well, but we had nothing else to cater to the Classic gameplay style until then (Sonic 4 was an insult) so it made more sense back then.
3D Boost gameplay can work too, by expanding on the more expansive levels of Generations I believe it would satisfy a lot of those fans, granted we've only seen 1 boost level - which didn't look much different to 3D Green Hill.
There's no way Classic is coming back in the next Modern game. Sega focus on making Modern Sonic a AAA experience too. Taxman and co. have this formula sorted.