1) A new weapon: The Soul Cube!
You know, it's that unique weapon from Doom 3 that you can use after killing X enemies, then you can throw it and it sucks all HP from the nearest enemy and gives it to you.
Could be nice as an artifact weapon or a unique?
2) A "Time left" counter for berserk, invincibility and envirosuit. Because who didn't find himself into a situation where he was rambo'ing around because he was invincible anyway, then suddenly you become vunerable again... Or you're taking a lava bath and all of the sudden your envirosuit wears off? The "Time left" counter would make you able to at least time it a little bit...
3) Invisibility Globe! You pick it up, enemies don't see you until you effectively attack them. Could be handy for AoB or AoPc.