My current plan is to build a Heroes of Might and Magic style affair. It translates perfectly to a dungeon, just replace dungeon with continent, level with nation, and room with province and the logic holds true.
After one thousand years, the ancient lich has broken free from their black prison beneath the earth, and now hordes of undead swarm across the land once more, surging up from the ground and taking the realms by surprise and subjugating them in a manner of days. You and your companions, a band of mercenary commanders, are in the eye of the storm of conflict, where the undead are there weakest. Can you muster your forces and drive back the legions of the lich from the free nations, then take the fight to the underworld and vanquish the ancient evil once again?
Trying to hash together a custom system, combat is pretty clearly going to be its own minigame on a small grid of 12 spaces or so. Big emphasis on the unit defining your primary combat abilities while your commander is more utility and out of combat stuff. Additional units would be more powers than additional bodies on the board to keep gameplay from just turning into a wargame and make stuff flow faster.
There would be multiple nations to free with all kinds of locations, units to recruit, ancient treasure to find and allies to make. Eventually doors to the underworld would open up, allowing sojurns down into the lich's own realm to take the fight to the heart of their dread power. Players know some broad landmarks at the start, capitals and such, but plenty of surprises to discover along the way.