NPCs with their own moveset or weapon loadout,could be a great idea!

Serious Sam:The One and Only Hero
Zorg Zotty:Fights with you while using weapons he stole from Mental's forces!
Peppino(Pizza Tower):A cowardly,yet quick and brawn pizza cook! Also uses a shotgun and a spell book.
The Noise(Pizza Tower):A crazy Super Zero who tries break into pizza restraunts! Can use a minigun and a jetpack!
Dream Wanderer(7smoke Tennis Blue Campaign):Uses weapons in from the popular custom campaign!
Hudson(7smoke Tennis Red Campaign):The Mighty Space Bounty Hunter,his weapons are all sure some Sci-Fi mumbo jumbo!
Tracer(Overwatch):She's nimble,welds 2 Pulse Pistols,and can reverse herself back in time!
Isaac(The Binding of Isaac):Uses items that can affect his tears,even a Revenant's skull as a Flamethrower!?
Fiend(The Binding of Isaac Repentance,Fiend Folio Mod):Uses his own minions to attack,along with Fireballs,and some Fiendish Foes as weapons!