The Legion of Superheroes from the latter three editions of Dungeons & Dragons (which were deliberately design to undermine several actual play features of previous editions) have given scant attention to domain games and hex crawls, in part because the implosion of resource limits makes them an exercise in futility. The other part of it is that managing an estate and running a war on orcish bandits takes too much work for players who just want to watch their bonuses stack to infinity.
Almost all of the hex-crawl and domain game material produced for D&D since 2000 has been the work of OSR creators, most notably Sine Nomine. Domain and mass combat rules are available in Dark Dungeons (a BECMI/Spelljammer simulacrum), as well as some content in OSE (modeled after the BX rules) and form a central component of the Adventurer, Conqueror, King System. This last has extensive domain design tools as well as an expectation that players will eventually found their own strongholds and become regional players. As such, it is easily the best integrated domain game system on a Basic/Expert D&D chassis.
Second place would have to go to the Red Tide set of books by Sine Nomine. It includes Red Tide itself, which is a setting book as well as an excellent, asian, sword-&-sorcery hex crawl toolkit. The book An Echo Resounding provides rules for designing domains. It has tools which create tensions and events within the domains, in ways that will create grist for your campaign or hexcrawl long before players start rubbing shoulders with Dukes or managing their fortified manors. It also has a rather distinct design philosophy than ACKS: instead of using HARN-style detailed peasant income statistics as ACKS does, AER has a top-down approach. They both generate authetic, usable B/X style baronies and city-states, and which you prefer probably depends on your needs as a DM. One advantage of the somewhat looser and less integrated Red Tide system is that it can easily be bolted onto other D&D games, and it can more easily digest bits of domain or hexcrawl systems (such as the Populated Hexcrawl series) to cover things not dealt with in Red Tide.
There is a free book called Worlds Without Number which has similar setting design tools, but it's focused on a Dying Earth style world.
The D30 Sandbox Companion is a great all-around resource for generating hex content and the occasional random event, but has no tools for domain building and is fairly rules-neutral (albeit clearly D&D-inspired).
The Old-School Essentials Populated Hexcrawl series is excellent, and it has both domain and hex-crawl resources. It covers some things not found in Red Tide, such as weather and meteorological event determination, broken up into several small books. Although my Western Lands campaign used An Echo Resounding as a basis, I still found several of the OSEPH tables useful to fill in gaps and generate thieves guilds for Slateholm.
OD&D and its simulacra have several rule systems which can be used for mass combat, but relatively few domain game supplements. Burgs & Bailifs and its sequel Burgs & Bailiffs, Warfare Too are probably the best OD&D domain game resources. While I haven't seen many hex crawl systems specifically designed for OD&D there are several hex crawl settings for Swords & Wizardry (specifically, the Hex Crawl Chronicles series).
there doesn't seem to be anything here