The D&D endgame has always sucked (except for *that* edition)

In the grognard-sphere, you can find many examples of grognards decrying the loss of D&D's endgame.

As I am working on B/X-ing 5e for Into the Unknown,  a cursory look at end-game approach is also on the menu (though mostly for a later Companion supplement since the core will only go to 10th level).

I've never really played with domain and stronghold rules. I was certainly aware of them and of the fact that the game was supposed to move in that direction. I just didn't understand how non-wargamers would think they are anything but an exceptionally boring endgame.

"You have over countless sessions fought everything from orcs to dragons, progressed from saving villages to saving kingdoms. Now, as you move into high-level play, new destinies and high level rules appear. Forget about resource management of rations and arrows. That's for noobs! At high levels, you get to manage the resources of an entire keep! Track the cost of building a new wing of the stronghold. Retain reeves and chamberlains. Collect taxes. Explore the intricacies of domain resources and incomes. Hear the complaints of peasants and track the cost of holidays. Graduate from low-level hero to high-level administrator! Play the domain game."

If that was the kind of campaign I wanted to play, I'd have gone for Birthright from level 1. The domain game fading from D&D was a natural consequence of it having nothing to do with the kind of game you thought you'd be playing when playing D&D.

That doesn't mean it was replaced by anything better though. 3e seemed to just go with the assumption of "why should high level play be any different from low level play? Just add more hit dice!"  Ignoring for a moment how awful the execution of this was in 3e, this assumption is, in itself, not terrible, since this is, after all, the kind of game you are playing D&D for.

Freebooters of the Frontier embraces this conclusion by declaring a simple endgame condition: amass 10,000 silver pieces and you win the game. This is perhaps preferable to the 3e's approach of "just keep doing the same thing with ever increasing numbers and levels of complexity." If D&D is to have an endgame, it should be somehow different, or at least an evolution, of the basic "kill creatures and amass treasures to defeat the villain" model of D&D. And preferably one that doesn't hand me rules for pricing the cost of holidays in your barony.

5e is not much better. Basically, the only edition who seems to have gotten the right idea is 4th edition (drops truthbomb, runs away).

4th edition D&D introduces epic destinies. They come on board about 8-10 levels too late, but they play to a good assumption of high level. While low-level play is often reactive and about a party walking the land waiting for destiny to happen, high level play ought to be about crafting a destiny for yourself.  Epic destinies enable this deliberate approach towards building your character into something larger than life the way you want to be larger than life. Low-level D&D play is "choose your own adventure", high level play should be "choose your own destiny." and 4e is the only edition to really grog to that.

In terms of flavour, 4e often does it quite well, presenting a good load of destinies that are essentially about transitioning from mundane hero to genuine mythic paragon. From legendary generals or sovereigns, becoming an archmage or epic lorekeeper, to questing for demi-godhood, heimdall-esque defender of a cosmically significant place, or an entire people, becoming consort to a deity to emerging as an actual avatar of a deity, thieves who can steal actual concepts, to becoming a literal parable yourself.

A lot of this is very high level stuff of course (and oftentimes more a culmination of a path of estiny) and some might want some intermediate high level play, on the level of domain play, becoming a guildmaster or similar. That is sort of what the Paragon tier (levels 11-20) was supposed to accomplish for 4e, except the execution amounted to little less than "choose your next splash option".

But the point remains - In either case, high level play should be about crafting your destiny and the rules for high level play should be in support of this - Rather than assuming that the end game should revolve around one goal (domain play) or none at all (3e).

So basically, the high level endgame I would want to introduce for Into the Unknown has very little to do with B/X and its endgame (cosmic adventures, a la the Mentzer Immortals set, would probably come closest as a possible destiny to choose from). I don't think I want to take an old-school approach just for the sake of doing it old-school.