-Sniper Turrets would be seriously spam gay tastic. LOL it would be like having 13 aimbots.
-Gas Turrets are spamtastic as well, but fun. I played in this one server that was playing a really simple small map with "man cannons" that fire you around. Hide Gas Turrets everywhere and the ground becomes foggy! I'm okay with them although they are definitely not good for comp. play.
-There needs to be a low grav mutator.
-ELD: Extended Lawn Darts!!
-NoEnergyMut: No energy depletion, therefore turning XMP into more of a 4 flag CTF match. And turret fest. There should be a configurable limit for deployables.
-FullAmmoMut: Infinite Ammo. Some restrictions apply.
-NoArtyMut: No artifact nodes, therefore turning XMP into an energy survival battle.
-ArtifactCombos: It would be cool if you could temporarily "combine" artifacts to create new vehicles or superpowers or something. 3 players carrying artys could "merge" into an energy entity or something.
-Jetpack: OMG instantly turn XMP into Tribes, lol.
-AlienGuns: Add the Drakk and alien weapons!!! (i.e. Spider Gun / Drakk Laser)
-Alternatively, you could create a new class.
-HeadshotOnly: Headshots are instant kills, and are the only type of hit allowed
-InstagibSniper: Instagib Sniper Rifle
-InstagibRocket: Instagib Rocket Launcher
-FasterVehicles: Self Explanatory
-SuicideRaptors: Remember this mutator? LOL.
-FoggyZone: Extremely Dense Fog fills the map. Artifact Carriers and vehicles light up the fog like lighthouses.
-Doom3Mut: Complete darkness. Everyone gets a flash light they can switch on/off.
-BottomFeeder: Lowest Score player gets random speed/strength/defense/etc. boosts
-UTXMP-RPG: Ever play UT2004/03 RPG?
Those are my current suggestions...