UT2k4 is my personal favorite shooter of all time as well. It's the only game that I actually played seriously competitive and even played (and won) some tournaments at big LANs back in the days when 300+ people LANs where still a thing
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Maybe after Tribes, another classic, has been revived now, it really could be time for another UT as well. However, there would be certain changes that I think are needed to modernize the franchise:
1. business model: a "comebacker" such as UT that doesn't have the mass kiddie fanbase of a CoD behind it would have to go F2P + microtransactions, which is not inherently bad if they are merely cosmetical or for certain comforts - any sensible f2p mp fps developer will avoid a pay-to-win model as hell.
2. Nowadays, and I'm not any different here, people expect some kind of progression system like levels/ranks with small benefits and/or unlocks. Personally, I'd love to see something similar to the LoL rune/masteries system that only give small bonuses.
3. Can they make a new UT based on the new U4 engine accessible to more people than just those with a high-end PC? Battlefield could get away with it because it still had a huge fanbase but for UT that has been presumed dead for some time now it would need every possible player it can attract.
What I obviously wouldn't change is the general gameplay. Have (mostly) the same weapons, base the movement on 2k4 and I'd be happy