I understand why many people disliked 4e, and some hated it, and I'm not expecting anyone who hated it to start loving it. But by the same token, 4e had some good ideas that can be of use in the future, even aside from the elements that 5e adopted but refused to call by their 4e names (like hit dice).
Bloodied. Best mechanic 4e gave us, and it's not even close: a mechanical reflection of how a creature might change as it gets more and more hurt.
And the 'bloodied' condition doesn't always have to kick in at strictly half a creature's h.p. total. It could happen as soon as the creature is damaged at all; or not until the creature is close to death...each one could be different. But the idea of a creature gaining (or losing!) abilities as its health gets worse is excellent.
I personally would like to see the return of the Warlord. Playing one was the most fun I have ever had in a DnD campaign, and the 5e pseudo-versions just don't do it for me.
As long as it doesn't have (1) non-magical and-or (2) ranged healing, as those concepts do need to die in a fire.
A fire which I'd be quite happy to light.
I would also like to see the return of clear rules (rather than 'rulings' and DM fiat), especially for things like Stealth.
The problem there with stealth is that no set of rules can possibly cover every situation that's going to arise during play at any given table (never mind every table!), so either the in-play situations have be unrealistically shoehorned to fit the rules somehow or the stealth rules have to be 58 pages long.
I would also like to see the return of negative hit points from still earlier editions. The way characters currently keep popping up in battles, going from 0 hits to 1 hit and back again and again, makes 5e combat seem like a silly game of whack-a-mole.
On this, however, you have my absolute support and agreement!
And if they don't bring back negative h.p., another answer is some sort of rule or system that severely limits what you can do for the first few rounds after being cured up from 0. Maybe a scaled and unmodified die roll, something like:
1-2 - you remain prone and cannot rise. You are defenseless.
3-5 - you remain prone and cannot rise; you get the benefits of your defenses, and attacks against you have advantage. You can do nothing other than defend.
6-10 - you can move at 1/4 rate, all attacks are at disadvantage, melee attacks against you have advantage, you have no reaction or bonus action this round, and you cannot cast spells of any kind (but can activate or use devices)
11-15 - you can move at 1/2 rate, you can cast a spell or activate or use a device, but you have no reaction or bonus action this round
16-20 - you are fully functional and can act as usual.
So if you got cured from 0 you'd roll against this table each round, at +1 for each previous roll, until you got into the 16-20 range; and a subsequent roll cannot make your situation worse (e.g. if you roll 12 in the first round of recovery then roll 1 in the second round, the 1 is ignored as your situation cannot get worse) until and unless you get hit back down to 0 again, which starts this process over.
Note this is an off-the-cuff shot at this; were I designing it for real it'd be a lot more granular - probably a different condition for each number below 16.
I would also like to have something for my character to spend hismoney on, and magic items become more abundant. Yes, I know, that raises some balance issues, but 5e already has balance issues, at least judging from the published modules (which are TPK-fests). On the subject of balance, I'd like to see more of that too -- how about fewer Kobayashi Maru scenarios to start?
I don't mind magic being more abundant as long as it's also made more fragile.
As for "TPK fests" - this rather goes against what I most often hear about 5e, that it's too forgiving on the PCs.