Well, first I would like to remember that Mustafar does have plains... You can make nice and easy ash plains with perhaps some lava rivers in the limits (in the part you cannot go).
But onto your map, as it's the idea you wanna develop now, I think it's cool.
Maybe you should take ihat bridge like those in Mygeeto (on Episode 3), making it broader so that a big part of your soldiers can get through. Or maybe, apart from that, even adding secondary bridges.
Another idea I have is that you could add some of these in the map limits or somewhere, as they are very Mustafarian:
About the buildings: I could find some pics you can use as reference:Entrance of a Mustafarian buildingForgotten crashed ship from the Old Republic
(great prop for a Mustafar plains battle scene)Entrance and bridge