Give it heavily increased tracking, and make it so that it regenerates ammo over time. Also enemies killed by the projectile creates a huge damaging explosion, kinda like the effect that Sunshot has.
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There's definitely some fun things a catalyst could add to PvE, perhaps a slightly increased blast radius or projectile speed. Even something simple like deep pockets or auto loading holster. Given the gun's status as the PvP bogeyman, however; I doubt it'll get any meaningful upgrades.