Absolutely. Maybe not Birthright, but right now I'd *really* like kingdom-running rules for PCs, including population growth, resource management such as forestry and mining, taxation rules, upkeep/infrastructure rules, and "general happiness" rules/modifiers for the populace (from "fanatic" to "revolt", for example!). And, of course, a d20 mass combat system.
I don't know about anyone else, but at 15th level, my players get pretty tired of constantly beating on monsters, and while exploring the Planes (and spelljamming, etc) is a blast, too much of it can result in some monotony. They want to run their own little barony, starting from scratch (build their stronghold first, then build a road to the nearest trade way/settlement, then have an inn or tavern spring up, then a village, etc) - during which time the PCs still have to adventure to defend their little realm (opportunities for both dungeon hacking and political adventures with neighbors).
I'm trying to (slowly) develop the rules myself, but I've discovered these days that I have way more money than time... :/ Ah well, wishful thinking!