What I wish would come back is really good COOP PVE. I ran a hugely popular ut2004 Invasion server that we wrote a custom mutator for (Sudvasion). I had a T1 line to my house that just ran the servers. We had a solid dedicated group of people from all over the world playing that 24/7. Was the pinnacle of gaming for me. 18 years later I still chat with some of the community via steam disc etc.
If you were an invasion nut then you likely did. We had alot of good tweaks for invasion and it was HARD. 100th wave was only done once and took 6 hours to make it heh.
And i agree. 2k4 was good in so many ways. Ive been searching for something like it and it doesnt exist.