Be creative here there are no wrong ideas.
I would like the Spartan laser to be a weapon personally, just called something different "Charge Rifle" was a name I had in mind
-
Grenade launcher. Wouldn't be as powerful as some of the guardian ones, but give us some interesting types, such as double bounce where you can try and pool shoot it over barriers and around walls. Sticky for tagging your foes. Mined for laying down the sneaky traps from afar. Give it something like, three rounds in a mag. So it would take three shots to take a guardian from full health to dead.