How difficult would a fanmade modular open-source engine for MTW style campaign be?

[caravel]

Personally I think any open source engine for STW/MTW should do away with the perspective plane and sprite scaling approach (the 3D appearance of the map and the scaling of the map pieces) and just go for a basic flat top down map with draggable pieces (yes something along the lines of what Garnier has developed or the paradox games). This should be fully playable without the battle map component, i.e. if the battle map is not present the option to fight the battle yourself is greyed out - autoresolve being the only option. The campaign map game need only run windowed, but apart from appearances it should function exactly like the STW/MTW campaign. The engine could still use the same MTW map textures, pieces and other graphics and sounds - so in essence kind of like a flattened MTW with no DirectDraw, running only in a window (I would put together a mock up if I had the game installed and if I wasn't at this moment at work).

The battlemap is a whole other project. That's the real "total war" and where most of the code probably is. There may be existing open source 3D engines that could be adapted to recreate it - all that would be needed would be the textures and models from MTW, which the player would need to buy. In these types of ports a lot of improvements are possible, such as 3D models on the battlefield instead of sprites.