So there's something that has been bugging me for a bit. Since Beta3, actually. There are always bigger fish to fry, but it seems like things are getting to the point where bringing this idea up can be productive.
So, the big reveal, the grraaannd idea is... oh, it's in the title, nevermind!
You already have this, with bridges. They are Open terrain mixed with Water terrain, and it works brilliantly. It's very nice. Punish your enemies for leaving units on bridges using your swimmers, but beware their pikemen! Anyway, even back in Beta3 I was wishing for things like "wooded hills" and "Snowy forest". It would require some discussion of what would work, and it would mean some things would get (welcome?) rebalances.
No more multiple terrain bonuses for units. For example: cavalry would only get their native bonus in "Desert Plains", and none in "Deep Desert" (Dunes/true desert) or "Sandy Hills" (or "Rocky Desert" if you prefer). All movement attributes should be fully preserved in overlapping types, but I would suggest that individual combat bonuses be 50%.
Everything would operate just as it does now with Bridges. Units would move across these locations using the best available movement option, just as now. They would give terrain bonuses in movement and combat just as now. I would suggest only that units gain only 50% of their individual bonus in overlapping terrain. This should probably be implemented on bridges anyway, as a Pikeman shouldn't get a full bonus trying to fight a Kraken from atop a bridge, and neither should a Sea Serpent get full bonus trying to snatch soldiers from bridges. You could even have "Snowy wooded hills" if you want to go that far, and it shouldn't disturb anything. It's not as cut-and-dry as the current system, but it's still easy to comprehend and it's far more organic in mapmaking and in the strategy portion of the game.
So, this would be expensive to implement. But I'm ready to support it with funds! (When the current run is over, which I'm sure you are all participating in as well!)