There's a lot of stuff I'd want back, but to keep things simple and realistic the ones I would bring back:
1- Actually SHORT short rests. 1 hour is too long. 10-15 min, the basic times of most rituals, works well.
2- Interesting Monster design with roles built in to make encounter design more dynamic and easier to do.
3- The Bloodied condition and all its potential hooks.
4- Mark could also be brought back as a mechanical hook similar to the Bloodied condition. It would help communicate intent very well.
5- Skill challenge framing
6- The Disease track! It was really an easy way to model complex curses, diseases and even long lasting injury. You could have a 'broken arm' and 'broken leg' track, for exemple!
Also, if we can't have Healing Surges back, I'd like to see more stuff that drains HD outside of the just healing in short rests.
I like that. The only similar thing in 5e RAW that comes to mind are creatures that can take additional actions when they've dropped a creature to 0 hit points, which doesn't happen that frequently with 5e PCs to see a lot of use. Mainly I've used it in a "save the villagers" scenario where, e.g., gnolls are rampaging through a village. When they down a commoner and immediately attack another it creates a heightened sense of urgency. If similar things could be done with the bloodied condition, it would force PCs to be more liberal with spell slots and potions for healing.
The one that always stick to my mind is that Dragons automatically recharge their Breath Weapons and use them as a reaction the moment they get bloodied! (4e Dragons were more fun than just 'Two claw attacks and one bite VS Breath weapon'). It made fighting them way more dangerous.
Also, I'm pretty sure the monster stat block told you the bloodied value as a quick reference? If not, it should.
Toll the Dead would be a smaller text block if it could just reference the bloodied condition
Less a specific thing ... but I want OneDnD to get some of the more inventive and exploratory energy that 4e had. What is missing from 5e is some of the more experimental stuff that 3.5 and 4e were willing to do. Incarnum, Shardminds, Avengers, Warlords, the last time I saw something that felt even a little of that was the discarded Mystic class.
Races like Shardmins and Wilden were fun. Class wise there was some fun stuff, like you say: Warlords, Avengers, Invokers, Warden, those were really cool. The 4e Monk was way better too.
The Mystic's problem is that it could easily have been sliced up into 2-3 different classes. It just tried to do EVERYTHING too hard.