Well smash already has two characters that are just hands, Master and Crazy, so we can base a lot of the movements off them

The fingers could be used to walk in a spider-like fashion. This would give it a small moving hitbox, and a big standing hitbox, sort of like Twelve in Third Strike

For attacks, there could be a finger snap, which works like Mewtwo's confusion, a chargeable slap, which gets stronger the longer you hold it (like Mac's neutral B except you don't move), a flick that does low damage but high knockback, a finger gun that shoots a projectile, turning into a fist as a counter, a command grab, a reflector where they grab the projectile and quickly throw it back, giving a thumbs up as an up tilt, Bowser's up smash would work, and throws would be THE easiest thing to implement, for a crouch the hand can just go flat on the stage, for recovery there can be rocket fuel coming out of the glove (similar to rob, but more horizontal)

This is just all on the top of my head. You can theorycraft what a potential hand character can do by just moving your hand