Not all of these are abilities, but have to do with adding variety to the character system.
First off, uncap the speed limit, but make it so that the higher you go, the more your acceleration/accelstart is capped, and vice versa.
Automatically going into jumping frames upon falling, and maybe while on a spring.
Instant death from any attack - however, turning this on automatically gives a character huge benefits - the runspeed/acceleration relation would be much less strict, and you can have a really high run speed and still decent acceleration. And maybe some kind of bonus in Match/CTF, though I'm not sure what. Maybe a higher firing rate?
This is just in general, but make it so that while carrying the flag, you automatically have Sonic's attributes, because right now there's no reason to be Tails while carrying the flag, and the only reason you want to be Knuckles is if you need the high firing rate, which you rarely do.
Meleeattack 0/1 - allows you to attack players in Match/CTF by spinning/jumping into them. Perhaps that one ability that lets you instantly break breakable blocks could allow you to simply touch players to hurt them.
A new momentum based thok type - It multiplies your current speed by actionspeed, in percent. i.e., setting actionspeed to 125 would make you go at 1.25 your current speed upon thokking.
A thok that can be aimed up/down. It balances itself out in Match/CTF, since you can't thok forward while aiming up/down.
Selfdamage 0/1 - You take damage from your own weapons, this will mainly be for Bomb/Grenade/Bounce, but I'm sure idiots will find new and innovative ways to hit themselves with Rail or something. If you get hit by splash damage, and you're jumping, you still lose rings, but rather than going into hurt frames, you take high Knockback and go into your falling frames.
A much higher cap on jump height, but if you go over the current max, then your actionspeed is lowered depending on how high over you went. If you go too high, then you lose your ability altogether.
Bring back ringslinger and slingitem, but make it so that enemies don't get hurt by custom slingitems past 64 fracunits. This allows people to use ringslinger in creative ways and for melee weaponry, without making it a ranged attack that can destroy pretty much every robot in SP without effort.