This post differs from previous "Games I Want to Run" posts, in that instead of contemplating what system to use, I can happily say that ACKS will likely be the weapon of choice for considerable future gaming. Instead, setting is the primary question.
So I think there are three main settings which have my attention currently, and which seem to have a tendency to recur in my thought even over the course of years. Those settings are Midnight, the Western Marches, and the Wilderlands. There are some interesting commonalities here. All three settings are pretty wilderness-focused; in Midnight, the wilderness is where you are safest from Izrador's law, while in the Wilderlands and the Marches, it's where the adventures are and an obstacle to be traveled through. All three are also fairly unforgiving; there are Dangerous Things in the world, and they want to eat you. In both of these regards, all three are well-suited to ACKS. Midnight might get a bit funny with the economics, but the others would work quite well.
As for the particular instantiations of the three, I'm thinking a Wilderlands campaign that starts with the sprawling dungeons beneath the City-State (inspired by Hill Cantons and kjd-imc) at low levels and gradually extends outwards into the wilderness towards classic Wilderlands locations like the Caverns of Thracia, Modron, and so forth, naturally all interpreted with a fair degree of creative license. For example, the word Modron, for me, is reminiscent of these guys, so I get mechanical vibes. In my Wilderlands, Modron is a mostly-human city which became a haven for heretical dwarven machinists cast out of Thunderhold. They turned it into the Clockwork City, complete with Rhodes-style defensive clockwork colossus watching the mouth of the Roglaroon, but tensions between Modron and Thunderhold remain high. Haghill sounds to me like it's ruled by a lawful (evil) hag countess who has sworn fealty to the (likewise evil-side of lawful) Overlord. And so forth.
For the Western Marches, I've been working on a Vikingy sandbox in the same model on-and-off in a number of systems over the last couple years. It would be interesting to run, but I feel like I missed my chance, and now it would likely be held up against Skyrim due to similar themes. So I might hold off on that one for a while longer...
And finally Midnight. Always tricky, Midnight. But Alexander Macriss made an interesting note during this interview, that ACKS' default setting is very much a 'crumbling empire in decline' sort of place, which reminded me of an idea I had for Midnight some time ago - run it at the end of the Third Age, with events in the campaign leading up to the Last Battle and the (probable? certain? prophesied?) defeat of the armies of men, elves, and dwarves before the Night Kings and their orc hordes. Start with hunting spies and cultists in the cities and their dungeon lairs, then move out into the wilderness in pursuit of orcish warbands and marauders. Finally, come domain-time, move into the political realm, try to maintain cohesion among the allies while ousting plots among the nobility and preparing for all-out war. Or fall to darkness and perhaps become a Night King yourself... Come the Last Battle, many PCs would probably die fighting, but ACKS' hench focus would leave a goodly cast of mid-level heroes to flee into the woods to fight in the Fourth Age, in a world that the players once shaped. Retaking parts of the Fortress Wall or fighting in the ruins of Nalford is more meaningful when these were places you once ruled. Player investment in the world across the gap of catastrophe. And of course, there's always the possibility that they might win... but victory is temporary, for how can you kill a god?
Also convenient about Third Age Midnight (Dusk?) is that events are in motion which are not immediately threatening to the PCs, whereas in the Fourth Age, it can be hard to generate sensible events without necessarily crushing the PCs beneath a pile of orcs. There is the problem of foreknowledge, though - if you tell your players "We're playing Midnight in the Third Age, leading up to the fall of the world to darkness as the ending of a chapter of the game", they're going to act very differently from "Sure there are rumors of the Dark God stirring in the North, but nobody takes that seriously. Every other time he's invaded, we've pushed him back; we'll do it again if we have to. In completely unrelated news, the Prince has disappeared recently." Also challenging to mix the sort of inherent plottiness of "Izrador is coming" into a sandbox game; could prep it as a timeline, or use some sort of world engine to track the progress of his agents and react to PC actions (philosophical question - if you build a world engine with a finite-time convergence property, is it railroading?).
Of course, the syncretic approach is also an option. An icebox where the PCs may accidentally awaken the Shadow in the North gives a sort of poetic justice to it, since it puts partial responsibility on them, or a game set in part of the Fortress Wall focusing on wilderness expeditions into Izrador's lands leading up to the war would work well too. Viridistani invasion could serve much the same function in a Wilderlands game, as a catastrophic event which shatters much of the social order and redraws borders. I guess that's something to consider, too - even if the PCs in a Dusk game do manage a feat like killing the Night Kings at the Last Battle, there are orcs enough for everyone, and there will be cleanup operations and reconquests for years to come. Combining all three would be even better, but would probably require some custom setting work (and it's hard to justify a megacity + underdungeons right on the northern frontier).
Anyway, fun stuff to think about.