So I just thought of something that personally I think would have made a big improvement out of Sonic Unleashed in my opinion.
Now I have nothing against Unleashed I think it is a great game, but it was sort of the point in which Sega did want to just focus on Sonic himself so as a result non of the other beloved and recognized characters were anywhere in the game or story (with the exceptions of Tails and Amy who even then aren’t playable or contributed to the story all that much). Even Knuckles the protector of the Master Emerald and is quite knowledgeable about the Chaos Emeralds couldn’t even lend any kind of help for Sonic and Chip trying to recharge the Emeralds.
Now I’m fine with them wanting to focus on Sonic and his Werehog stages, so a decent compromise would be to have the other characters show up in different locations that Sonic visits sorta like they are doing their own sort of things and Sonic just happens to run into them which would be a nice way to still contribute to these characters still being relevant in some matter of speaking.
So a few examples of what I mean by this include:
Amy showing up in Spagonia doing some site seeing
Cream and her mother Vanilla in Apotos having brunch
Espio in Chun-nan doing recon work (he is a ninja after all)
Big in Holoska doing some ice fishing
Rouge in Shamar on another treasure hunt
Vector in Empire City doing his detective work
Charmy in Adabat having fun even though he’s supposed to be on an assignment
and Shadow in Mazuri keeping to himself, meanwhile Tails and Knuckles could actually contribute more to the actual story.
Anyways those are just my thoughts on the matter.