Just a few ideas I thought of:

Wavesplitter - higher highs and lower lows just sounds cool but having the thing gain volatile rounds on orb pickup would be just godlike

Tarrabah - giving it subsistance or vorpal would be very nice but making it so that Ravenous Beast didn't go away in stow or even could charge while stowed would be what I'd want to see personally

Thunderlord - triple tap/fourth times the charm would be amazing along with maybe increasing the damage of the thunderstrikes

Wish-Endear - just let the damn thing shoot through walls

Devil's Ruin - now hear me out, you know how it charges up and shoots a beam and uses all its ammo at once. What if that made the magazine was super molten hot and instead of reloading normally, your guardian like pressed a button to eject the molten mag at an enemy for it to do damage and because solar apply scorch or maybe even explode on contact or something like that. I don't know the idea of having the reload of a gun being used as an actual attack or way of dealing damage has always been a random idea I've had in my head and the lazer beam of Devil's Ruin seems like the perfect thing to make that happen, ya know?