Basically what the title says. We spend a lot of time in instanced missions, so it might be nice to see some more diversity to these familiar environments. But for a bit of extra challenge, perhaps we should also figure out how we might best go about building these maps, whether using existing assets or just what assets might be needed for something to be made.
My own big suggestions, and possibly the easiest ones would be more transitioning maps like follows:
-Caves, sewers, and tunnels into any types of tech map tiles (such as praetorian power, biological, or normal tech, abandoned tech lab, normal tech lab, etc.) by connecting these with a simple door, it can be used to give a sense of going from an out-of-the-way spot into some hero or villains lair. You could also probably include things like warehouses and offices with 'hidden doors' that transition into those and things like Arachnos, Fifth Column, council, and Longbow bases
-another type of map that might be nice to see with a minimal change would be various outdoor maps such as ruined city and the three forest maps sans the war walls. These would greatly increase their flexibility to be many other places in the world. Similarly, perhaps Croatoa outdoor maps without the gold filter.
Those are the big ones that come to my mind. Actually have a few more but they'd take longer to post details for.