I think they could all get something. They don't at the moment, but I think they all should.
TB: Fire could just be a damage explosion, ice could be an area slow.
Rook: He falls on you and crushes you. Smaller area than TB's fire death, but does more damage.
UB: Giant acidic explosion. Perhaps give a plauge-like effect instead of doing direct damage?
Regulus: Give a slowly explanding LoS as he rises above the battlefield. Would be useful to allies in the area.
Erebus: Leave a slow, lingering area damage. Similar in function to TB's Ring of Fire.
Oak: Gives nearby allies a slight stat boost.
Don't know enough about Sedna or QoT yet, but I think they could all add a neat effect on death. Any other thoughts along these lines?