A few ideas for what to replace thok with in Sonic's moveset:
1. Parkour: This one would probably be a pain to script and would require the most new sprites, but I think the end result would probably be worthwhile. The general idea is inspired heavily by Sonic Lost World. Sonic can grip against walls while maintaining his forward momentum by holding jump, losing nearly no height at first but losing height faster the longer he holds on. Letting go of jump launches him off. He can then proceed to grip another wall if one is nearby enough. He can also grab onto ledges that are just barely too high to jump up to and vault himself up.
2. SA2 style bounce: Can be used to gain a little bit of extra height over what's possible with a normal jump, and can launch pretty far by bouncing off a badnik. Retains forward momentum, so however fast Sonic is moving, he will keep moving forward that fast while bouncing.
3. Wall peelout: Sonic can grab onto a wall and charge up a peelout to run up it. If he doesn't have enough speed to make it to the top, he will simply fall back down.
4. Slow fall: Sonic uncurls mid-jump to ride the wind, causing him to be vulnerable to enemies but allowing him to gain extra horizontal distance in his jump. While in this state spin will cancel, holding down jump will fast fall. Basically, it's the same thing as in Unleashed, Colors, Generations, etc.
5. Jetpack thok: Sonic pulls out a Jetpack Tails made for him and uses it to get a quick boost. Works similar to thok, except it can be used twice midair, once forward and then once backward, allowing him to correct himself. Although he's not curled up into a ball, he still damages most enemies.