StarFox's current problems lie mainly in the execution. StarFox is known mainly for it's corridor Arwing gameplay, and we've been getting less and less of this as time goes along, which in essence ruins the feel of the game completely. It's not as far off the road as other series though (Sonic immediately pops into mind >_>').
I don't think there's anything wrong with taking Star Fox outside of corridors (who DOESN'T love SF64), but it should still remain within the ship (bad, Assault, bad!). Also, if they are going to focus on "all-range" the way SFCommand did, they need to realize that arcade-style controls don't cut it outside of corridors. Besides the lack of a traditional option (bogus), I was very disappointed with the fact that you cannot manually roll your ship, AT ALL (even the SNES allowed 90-degree tilts).
Also, keep the speed up. Even Assault's ship segments weren't that great, they were too damn slow. I played the game again and again in store demos trying to convince myself to buy it, and I just couldn't. We're flying sci-fi jets, for crying out loud, they're supposed to MOVE.
As for Sonic, well, we could make a whole thread on that alone. Here's some pointers, all things taken from the 2D era that can easily be applied to 3D, and yet for some reason aren't:
-SUPER SONIC IN STAGE REPLAYS. If you cannot do this then GTFO.
-All stages should fork.
-Extra characters are fine, but their abilities should be useful enough to set them apart (offender: the varying teams in Sonic Heroes) WITHOUT disrupting the core Sonic gameplay style (offender: Were-hog). Think Tails in SA1 except without the mandatory short-stage race.
Every character should be able to play any stage (barring the mandatory Super Sonic final boss). Character-exclusive forks within the stage, however, are acceptable.
-Feel free to re-use stage textures if it'll help, but DO NOT tell us that a 3D game exclusive to speed-type characters cannot be done. Heroes and Shadow already proved otherwise, they simply failed in their execution.
-Never underestimate stunts, regardless of whether or not they involve player input. We still like our loops, corkscrews, running across water, etc.