Rocket would be bad-ass, like the raccoon from Guardians of the Galaxy!
You guys have probably gone through these names and their shorter forms / nicknames already but I'll just spit some your way: Michael, Anthony, Vincent, Eric, Thomas, Trevor, George, Freddy, etc
Maybe you (both mama and papa kryptik) have a musician, actor, artist, writer or a character you both love, someone who inspires? Do they have a nice sounding name?
Or maybe something foreign or viking-y? Silver? Ragnar?
My kid's name looks and sounds quite foreign compared to all these but is just a more germanic version of George. (We thought of it when I was still pregnant and we were sitting at a bar, I was drinking tea or non-alco beer, doesn't really have a deeper meaning to it, it's just hard to twist it into an offensive nickname + we don't know any assholes with that name).