For example- One thing I really enjoyed and missed about my Unreal days of yore, the Dispersion Pistol. I used to love scanning maps for dropped pistols or pickups so I could grab them and watch my weapon mutate and become stronger. I always though that was cool. The Phazer from the old Unreal4Ever mod for UT had the same effect on me.
And for a game type, I abandoned Quake 3 Arena quite a while ago, only returning every now and then to join my pals in a game of Urban Terror, but that's another topic. One of my favorite parts of Quake 3 Arena, specifically the Team Arena expansion, was the Harvester gametype. It was the Redemption Holy Skull Cross in CUT2 that reminded me of this. I always enjoyed watching people with many neon skulls trail behind them, and then stealing them after I killed them. Then making a mad dash for the deposit point. Great fun. You can always add a team aspect to this by making ONE blue deposit point in the red base and vice versa. Keeps things interesting.

And while there are separate mutators/mods for these, I would love to see a Chaos take on them. The Overload game type (firing upon an effigy or crystal in the enemy base until it breaks, to win the game) and a Kamekaze type power up (or Redemption-rune type for UT fans), where a giant blast will rain down the hurt on nearby enemies after your own downfall.
Just some thoughts. :p
~Apok