I don't think they know how anymore...
They could start by watching Jim Sterling's video on the subject where he points out what should be obvious: that whenever they've found good approaches to gameplay, they've immediately abandoned them for future games.
Another one would be not to loan your main franchise out to random third parties.
They should just make Generations 2 and people will pay attention again. Granted, it is a hard game to get working in three dimensions. But both Sonic Adventure games and Sonic Generations had moments of brilliance. They just need to take those, build on them, and remove the flaws.