Well...
--The focus would be on the Tournament again, no silly SP storylines, just a bunch of criminals, fortune seekers & wannabe heroes fighting to the death with all sorts of crazy-ass weapons in crazy-ass arenas.
--Advanced locational damage for both players and vehicles. Limbs & wheels could be shot off, for example, crippling (but not destroying) the player/vehicle.
--Advanced bloodloss system, with blood & healthloss depending on the severity of one's wounds.
--Realistic, really disgusting gore.
--Advanced species system. For example, not all weapons would be equally effective against / have the same effect on different factions. Think EMP weapons against robots, UV based weapons against vampires (yes, vampires.. they would replace the Necris) etc.
--There would be dozens of unique, cool, weird & wonderful characters (think for example Open Arena) spread out over at least 4 factions / species. These would be made by many different (community) modelers on freelance basis, to stimulate quality (every model being someone's 'masterpiece') and stylistic diversity.
--All female chars (50% of human faction) would be sexy and scantily clad, with realistic breasts.
--Normal looking guys/people instead of anabolic tank persons.
--Anatomically correct nude skins for all humans and some aliens.
--Highly advanced voice packs, with a unique voice for most characters, a crapload of (sometimes VERY 'mature') taunts, and all sorts of situational feedback. Bots would apologize when shooting someone on their own team, praise other people's skills, and sometimes even beg for mercy when wounded / cornered. Also there would be realistically horrible screams for horrible injuries, and bots would say silly stuff like 'Help me, Jesus!' when running around on fire and such.
--The game would come with at least 40 maps, all made by different people (mostly from the UT community) on freelance basis. Every map would have its own distinct theme and style.
--4 (or more) team support for all team-based gametypes.
--CTF would become Capture the Fag. And yes, you would actually be capturing fags.
--WAR/ONS would be scrapped and replaced with a new gametype where the objective is to either capture or destroy the enemy's respawn units, and subsequently wipe out the opposing team(s). Maps & matches could be completely asymmetric with for example one team attacking with vehicles (some of which have respawn units), and the other defending a base.
--Jailbreak would either be an official standalone gametype, or integrated into the above.
--The weapon set would be greatly expanded with both 'real' (contemporary) weapons and crazy sci-fi weapons. Laser pistols, hand grenades, assault rifles, Mongolian Deathworm launchers... think BW mixed up with U4E, and then some. These could be mixed & matched according to one's preference.
--Melee weapons with appropriate 3rd person anims and perhaps auto-switch to 3rd person view.
--Idle / unarmed animations, plus all sorts of unique anims for each character.
--Flak monkey and Holy SH!T would return, and be joined by many new titles/awards.
--The UI would combine the very best features from UT1 & 2, with, among other things, superb bot management and tons of customization options.
--Excellent mod support, including custom skins, voice packs, models, vehicles, muts, TCs, and of course maps. Easy WYSIWYG tools for n00bs, and advanced tools for pros.
--Regular reviews and rewards to stimulate modding. The best content would be released in bonus pack form, and the modders appropriately praised and compensated for their efforts.
...etc!