I think its sad that SEGA never really explored a slightly new way to play Shadow that still made him fun. The gunplay didn't work in my opinion. I think this is something that Nintendo does really well with their side games. They sit and try to come up with slight off gameplay decisions that make those characters feel unique and give them their own games.
Example: Yoshi's jump, tossing eggs and eating enemies. Kirby's suck and power change/flying ability.
I think Sonic Team should have made a platforming game based on Shadow, with time traveling and have a gimmick of using Chaos control. The issue with Shadow is that his gameplay is 1:1 with Sonic really and Sonic Team would have to focus on making him play different and fun, which just seems impossible.
Personally I rather have a Tails solo game that is more in the vain of like, Mario 64/Banjoo collect-a-thon since technically Tails makes gadgets. Would be cool to platform, explore and collect items to make cool new gadgets that you can use in a metroidvania type world that opens up slowly.
But hey... Shadow is 'cool', so whatever.