It’s not a needful, simple thing that would improve the game and it’s more a case for a mod but anyway:

In the context of ARMS coming out and having played the demo:
I’d love both mutations and bionics to make ARMS that power up and extend your punch range up to 5 tiles, similar to how the spear and bullwhip do.

That would be fun.
Something like a “Spring Arm(s) CBM”, and different hand augmentations(that would completely limit installing other cbms in that hand[?])
I don’t think it would fit that well in the context of the game, though. Still, it’s a cool idea.
That idea would also fit nicely in the Slime’s “Strechy Limbs” mutation.