Sonic Advance 3-esque Co-Op mode

I had an idea for a Sonic 2 hack that would have a Co-Op style similar to Sonic Advance 3, in which you choose your player character and partner character, to stray away from just (Character here) and Tails games. I would like to know a good way of approaching this so I can see how it would work in a proof of concept hack on just vanilla game without editing any levels.

 

The first thing I would do is figure out how to go about adding the new "partner abilities" that depend on the character following you. I'd imagine this is the most difficult task here, because it likely would require one to work with the AI and the spritework of the partner character, and figure out how to make them work in tandem. For this, I suggest looking at how Sonic 3&K handles Sonic and Tails' flight interactions. But yeah, I would figure that out first; the rest should be a simple matter of adding the characters you want, branching to different double-jump abilities depending on the partner, etc.

 

Also keep in mind the palettes, not a lot of characters can work within the same palette line. Tails tends to be compatible with both Sonic and Knuckles since he only uses their reds and skin tones which are identical, hence many hacks easily hack him in with other characters. Making Sonic and Knuckles work within the same palette will be harder but not impossible (as shown with Knuckles in Sonic 1 and Classic Heroes).

Hacking in other characters is even more intricate. Vector and Espio have entirely different palettes altogether, not even the greys are the same. Mighty would be easier but his shades of red are slightly lighter than normal, so you may need to have an alternate selection of them like Knuckles has. Charmy's palette could merge fine with Sonic's s1 colour scheme (a few odd elements such as his wings are connected to his blues so would alter with Knuckles and others).

 

For Knuckles and Sonic to share a palette, you could replace the two orange colors normally used for Tails with Knuckles' first two reds, and have the dark red used for Sonic's shoes as the third red for Knuckles' fur (unless you want Super Knuckles, then you'll run into problems). Additionally you could sacrifice one of the shades in Sonic's fur (or switch to his S3 sprites) to make room for the green on Knuckles' socks. This will cause changes to some objects, Buzz Bombers will have red and black stripes, and used starposts will flash red and... red.

So, for Sonic and Tails you can just use the regular art and palette, for Knuckles and Tails you can just use the regular art and palette, but for Sonic and Knuckles you'll have to use an alternative palette and alternative art for Knuckles (and possibly some other objects).