I honestly can't think of any decent arials for Geno. When I try I just end up with a mix of Mega man and Mii gunner.
I imagine there's lots of possibilities but here's some ideas:
Neutral:
-I've seen sets that suggest an attack using Geno's cape as he spins it hits in a circle around him
-Could also see Geno firing a beam, pellets, or stars while spinning in the air also with a circular hitbox
-Could be a quick two part attack like Samus' neutral air except using projectiles so hitting once in front and once behind him
-Could be a simple tuck and roll in the air again with a circular hitbox
-Geno whirl that spins in a circle around self instead of flying straight
Forward:
-Rocket punch in front of himself maybe even two simultaneous or one after the other (or one after the other with a timed hit)
-Similar to Ness' fair but just shooting stars or pellets
-Cape swipe in front of self
-Explosion from elbow in front of self (kind of like Zelda's uair but in a different direction
Back:
-Cape swipe behind self
Shoot a Geno beam behind himself at a diagonal downwards angle
-Elbow explosion behind self
Up:
-Hold up a star piece that deals damage
-Shoot stars/pellets above self arcing hitbox above self, could also back and forth like Zelda's up smash
-Elbow explosion above self
-3 stars from when Geno charges Geno beam appear over Geno's head 1,2,3 attack with bigger hitbox each time
-Reach into hat and throw and item up; item could be random with different properties and damage depending on item
-Rocket fist(s) straight up or one in each diagonal direction above self or something like that
Down:
-Shoot a big star or stick out a star piece below self
-Elbow explosion straight down
-Geno beam straight down
-Turn into cannon and shoot straight down great for damage and KOs but can cause you to self destruct if not careful
-Shoot pellets/stars in an arch below self
-Rocket fist(s) below self
Notice that pretty much all of these are interchangeable to some degree so you could really play around with them and I didn't include some of the interchangeable options simply because it was redundant. If they can make Terry's aerials different with just punches and kicks I don't think they'll have an issue making Geno's different.