DM would be best suited like Tribes Arena or DuelMOD, where you either spawn with your loadout, or you get to choose it from a menu.
Tribes Deathmatch was terrible, you should have spawned with maybe just a disc launcher, and have e-packs/other weapons around the map.
Maybe a way to travel fast indoors (something like increased player movement, only requires skill), either strafe jumping, double-tap dodging or a grapple would improve deathmatch greatly imho.
There was no real depth to Tribes Deathmatch, and it got boring fast..
I'd like to see mods like Rabbit, Hunters/Team Hunters, DrinkMOD, Siege, Duel, Team Rabbit, Arena (Similar to Rocket Arena 3 with multiple "Arenas" around the map), Defend and Destroy, Find and Retrieve, Capture and Hold all included in the game.
As for a new gametype, what about "Capture the Fortress", similar to Tribes 2's Siege, only a Light Armor only MOD, inside huge maze-like bases. Where you spawn with current required loadout (Shield pack/Plasma/Disc/Gren).
One of the main reasons maps like Scarabrae and Blastside suck, is because it's nearly impossible to kill a heavy w/ shield inside small corridors.
If it was Light only, no Turrets, limited Grenade Launcher Ammo (Maybe reduced to 5 or less), I think it could be really fun.
If you want to cater for the Quake players, do it with MOD's. It's a proven fact that one game-type can't satisfy everybody. Look for what's worked in previous First Person Shooters and build from it.
I also like the idea of multiple Arenas running different gametypes on one server.
i.e: One server runs 4 Arena's, of 4 gametypes, CTF, Duel, Team Rabbit and Find and Retrieve.
Say if you're sick of playing CTF for now (Which should have a maximum of 10 per team no matter what), you just hit TAB, select another Arena you want to play in, and you're instantly transported to another location on the map and playing a completely different gametype/map layout.
Servers nowdays can handle large amounts of players, and by the time the next Tribes is released, there will probably be 4GHz+ processors.
Anyway, back to Deathmatch, I think the whole idea needs to be re-designed, with alot more additions and modifications made to it, to give it more lastability.