Its a really rough idea, but here goes.
"An RPG/Civ game"
Okay, so the game has two modes. In "World" mode you choose a faction. Each faction has a certain goal. Stuff like destroy everything, bring the world under one religion, recapture a sacred land and revive a dead god, or just conquer the world. There are tech trees, and resources and stuff to capture. You can build cities, and determine the overall layout. Units cost certain amounts of gold/resources and require certain facilities to make. You make individual units, and fit them into squads. In World mode combat is a standard "rock paper scissors" sort of thing. Archers can plink at range, You need catapults to bust down walls. That sort of thing. However, Factions can hire out heros. Heroes are different from standard units in that they can level up and are generally much stronger than normal units. However, they are fairly unpredictable. They have there own goals and motivations. They can be loyal or they could ditch you when the going gets tough. You can give them titles, gold, equipment, and party members. Sounds kinda like your HoMM style games, right?
In the RPG mode, you make a character. Select its race and such, and choose what town you want to start it. From there it plays like a normal RPG, but the game has a calender. A certain number of "days" is a turn in Civ mode. And the things that happen in the "Civ mode" of the game, can affect the player in the RPG mode. They could suddenly find themselves in a city under siege. Or watch an army clash in the plains. They can be bystanders and just try to survive in a chaotic world(or just a mundane one), or activly influence whats going on. RPG mode is kinda of like a rougelike in that it only ends if the player dies(and that includes if a civ manages to pull off an end of the world scenario).
Basically RPG mode is you playing as a guy in a game of Civilization on autopilot.