A fire-emblemesque game where, rather then control a small group of plucky heroes, you're in charge of the mooks sent to stop them. You're given a bunch of "squads" on an overland map and are charged with killing said heroes with them.
The heroes will absolutely destroy these early squads but your pool of resources will slowly increase if you dedicate a few squads with finding resources in certain areas and throwing away a few squads at the party is essential to keeping them distracted and forcing them to go back to town to heal and resupply.
As the game goes on, you slowly accumulate more and more points while the heroes try to complete quests and get stronger, prompting you to send bosses to distract them while continuing to scour the world for more resources to throw at them. To win, you would need to either wipe out the party (very difficult to do until the late game) or spend a large amount of resources on a "dooms-day" clock to complete, or maybe just destroy a certain percentage of the world's towns and cities.
For the heroes to win, they would need to either destroy all of your spawn points or complete enough quests to track down the "final boss" and defeat it.
You could also be able to design your squads and even give them weaknesses in return for more points to spend on stats or cheaper recruitment costs, such as being forced to attack any hero in range, or being extremely weak to arrows.
I'd imagine that, strong or not, the ai for the heroes would have to be pretty good so they don't just suicide themselves on your spears like a mook-level ai would. They would need to know how to use the terrain, attack the right squads and when to fall back and heal up.