Alright, we're getting into the really obscure games now. Realms of the Unknown is essentially the D&D "endgame" and nothing else: Run a town or barony or kingdom and fight against others on a domain level. That's pretty much it. Note that I have never used this thing and I doubt I even read very much of it, so I can't really say if it is any good, but faoladh has a great review on his blog.
I have to say that I very much like the idea of this game. It seems to me that games like D&D that deal mostly with individuals, games like Chainmail that deal mostly with armies, and games like Realms of the Unknown that deal mostly with domains are each best dealt with separately, with different mechanics suitable for the different "resolutions" required.
In essence that's what BECMI D&D did: You call the result D&D, but really it's a losely connected hodgepodge of rules, different rules for each "level" of play. (If you ask anyone what D&D is they will not say "the war machine" after all.) So maybe it's better to modularize the rules, use independent systems, and then mix-and-match as needed. Of course that's really just a gut feeling, sadly I have never experienced those "upper levels" of play anyway, for the past 25 years it's mostly been dungeon delving for me.