Classic Sub-Zero - Snowball that works like Reptile's Slow Forceball. When it hits it won't freeze, only disable jumping for a few seconds on his opponent. This will help to setup his Ground Freeze.
Stryker - Add another move to the Baton Trip that juggles on hit. But make it so he is wide open if it is blocked or whiffs. Perhaps use the Baton Trip (with popup move also) as a dial-combo ender, but when it's used in this fashion, he is able to block any retaliation. Something like LK,HK,HP,fb+LP,df+LP and it causes DP.
Sektor - Have him call an Air Strike from his Tekunin Clan that sends Missiles down from the top of the screen, he can use Cyrax's Self-Destruct Fatality animation for this.
Also give the ability to detonate his Homing Missiles by doing Run,Run,HP to detonate first Missile or Run,Run,LP to detonate second Missile. That way you can detonate either Missile on screen depending on your opponent's location. This would also use one cycle of the Cyrax Self-Destruct Fatality animation.
Cyrax - Lower the recovery time on his Net so he doesn't get owned so badly. Make the Bombs detonate a few frames sooner to make them more effective. Make his Air Throw have Robot Smoke's range so he can throw people who have a jump kick out if it is timed correctly.
That's all I can think of right now.