my proposition:
"diablo walks the earth"
all players see a red circle on world map that changes position from time to time
the encounter holds some uber duper deathclaw with 99999 hp doing 100-300 damage a hit
he drops deathclaw horn - just nice item to show off, or it can increase some stat by +1 or to +10 when in inventory
Hehe. Another ex-diablo addict :-)
Promblem with that is, that the red dot will be a free for all trollfest.
On the other hand: it might be a brilliant TC substitute (short duration pvp where the biggest/best group takes the prize).
I would add to this suggestion: let the mega deathclaw be able to heal crippled limbs every so often. Or you'll have one slow moving tank getting shot from all sides.