Just make up an original name, keep it simple and try to keep it to one or two syllables (that aren't too tricky to pronounce.) It will help a whole lot for multiplayer callouts and even just general conversation. Avoid numbers, avoid filler (i.e. "Clan" tags or an "X" on either end), avoid alternating caps, and most importantly avoid pop culture references. AVOID POP CULTURE REFERENCES. You will get sick of the joke and eventually you will loathe every second of public chatter.
Asking for suggestions is the wrong way to go about it, the most important aspect of your gamertag is that it's a personal thing, it's essentially another name, not some clever title.