I like your idea, though it's similar to some other postings.  I suspect it's a feature they will consider 'advanced--include if time allows' considering SD is still working on the basics of the game in FE, but your idea has merit.

Two things they'll need to implement for this to be enjoyable: damage variance based on flanking and rear attacks; a tactical attribute for each unit. Perhaps scouts, invisibles, burrowers, flyers, etc. all get a tactical advantage and a slight damage modifier.

Some of this they seem to want to leave to the modding community (which I don't really understand, this seems pretty fundamental if you want the game to be uber-cool) but they have a lot of other elements that need work too (like the variety of magic spells).

Indeed you are pretty much limited to adding new variations of things that already exist. Want to add some new weapons, spells or traits, no problem. Want to make even to slightest change to how the game actually plays, get real. Basically you can add new content or rebalance things but you can't change even the smallest of game mechanics like caravans. So no, the modding community won't we doing anything of the sort.