I feel like the raw star power destruction for Geno doesn't work because of Bowser already being strength-based, as he's especially portrayed as in Mario and Luigi: Bowser's Inside Story. I feel the separable hands work well but I also think Geno could use star power to open cosmic-styled gates or perhaps magic locks with a tinker toy-styled hand since he's basically a Swiss Army Knife for guns.
I feel like it could ruin the character but I've always thought of Geno as being sort of mystic in nature.
I used the term a few posts back but I feel the best way to describe Geno is "GunMage."
It's also important to take into consideration the two aspects that comprise what Geno is as we know and love him:
1. A relatively powerful Star being, (he can turn a wooden doll into a body capable of keeping up with Mario in speed, power, durability, etc.)
2. A wooden doll that comes with overly-potent weaponry for a supposed child's toy. (What children's toy can knock Mario unconscious? Granted Mario was caught off-guard, but still.)
The doll part brings the guns, weapons, and surprises. The star-being brings the cosmic starpower that reminds you why he's supposed to be an "ascended" type of being. The Star power destruction is essentially "Star Magic" that can annihilate foes as needed. Compare the Incredible Hulk to Doctor Strange, who's stronger? The Hulk (Like Bowser) is more powerful with physical strength and martial might, whereas Doctor Strange (in this representation, like Geno) is more powerful in the mystic arts, which goes beyond muscles and force of impact.
Geno's non-doll abilities come from Star power. You know,
Star power? The thing in the Mario universe that grants wishes, invincibility, limitless energy, and can directly cause the creation and destruction of countless galaxies?
That Star power.