I think that Devil Bringer's ability to absorb abilities was a bit underutilized in DMC4 (you got abilities from Evil Legacy, Echidna and Credo, IIRC; I don't remember if Bael's ability amounted to anything - he got Rusalka's corpse, but I don't remember a new ability; EDIT: Bael's ability was "auto-search", and Nero also gains Anima Mercury, so three abilities from bosses (Echidna, Credo, Bael) out of 8 or 10 bosses, depending on how you count - a bit on the low side, fi you ask me). So you could have Nero get abilities from demons he defeats like Dante, except instead of making demons into Devil Army, Nero directly upgrades his own Devil Bringer abilities. Like, defeating Berial - ability to shoot fireballs and/or set himself on fire; defeating Sanctus - ability to be supremely annoying; etc.