I guess it depends how far they want to go with gems. The "safe" option would be to have gems only effect attributes like speed and stamina.
If they wanted to go all out, they could add in things like increasing your range, maybe an armour gem that decreases the damage you take when you get under 30% or something and walk through projectiles after you get hit by 3 or 4 in a certain time limit.
I'm not bumping the SSFIV thread so I'll put this here: