There are a few ways to create ossum battlefeilds, I did a hell of a lot of work with this a good number of years back, but most of that lego was given away/sold.
Anyways, you have to consider things both tactically and aesthetically.
If you are willing to use advanced techniques and a good number of parts, you could go for raised terrain. This creates rolling hills and plains, and is a great way to seriously spice up the average flat battlefield. Additionally, you can make trenches, foxholes, and possibly even underground bunkers! This is done like so:
If you don't have the parts or time for that, another way to utilize what you have is to do the following-
Have the raised baseplate in the center, and some form of structure on it. This is done to draw the eye towards the piece, and also creates an obvious capture and control point for both armies. Feel free to put some weapons crates and flags and shit up here.
Use grey baseplates and roads together, build as many buildings as you can manage without using rainbow(windows as firing ports, walls for cover, always keep in mind how the players may use this). Gas stations are great, 'cause they explode!
Put some civilians around here, have a few of their vehicles going down the road. having 1.5 vehicles per road plate is generally a good rule of thumb.
Drop all of your trees in the green areas, feel free to use whatever blue plates may be laying around to make some pretty streams in there. If you make it large enough, it can be an fun obstacle!
Keep BURPS near what is supposed to be natural rock formations, and build a bit on them to try to disguise their cookie-cutter BURP-ness.
Utilize those fences near the town, or if they are brown you can try putting them down near the jungle, and have that as a starting point for one of the armies.
Hopefully I've been of aid!