Quake needs more low-mid level monsters. Something in the 3-DBS-shots-to-kill range. You can only arrange ogres and hknights so many different ways before you want to eat a gun. Look here:
http://www.gamers.org/docs/FAQ/quakefaq/qfaq.html#8-1
There's a jump from the 60-80 HP's to the 200's. That's the perfect gap.
They don't need to be special, they can still be cannon fodder to any player with a nailgun, but their behavior should be such that you can't afford to ignore them while you take care of the more immediately threatening Shambler or Fiend.
On poison: the drowning mechanic, and the poison headcrabs in HL2, both have a great mechanic. They reduce your health but not permanently - it comes back over time. Momentarily injects the panicky tension of only having five health left without being overly cruel.
On monsters spawning monsters: how is this cliche when no quake monster has ever done this? the pain elemental from doom2 was swell, and had a way of also becoming a "high-priority" dangerous monster because you'd be swimming in other monsters if you didn't deal with it. Same with the arch-vile - resurrecting the things you already killed (and also being able to blow your ass sky high) made it a serious threat.
If Quake had something smallish and fairly pushover-y like a lost soul, this would be ideal. If spawns weren't so annoying the vomitus would have fit right in. (how about something that gives birth to scrags in mid-air?)