In my last RTS (senior year) I was happy enough for it to have no-terrain, and just be a networking Java project for unit movements and proximity related triggering.
I think I have better networking now. I did my first one entirely with UDP networking because the university LAN was so reliable. Now that I have TCP working, I will add UDP in for speedy state increments. I want to start the gaming side correctly. I could do the terrain in an X x Y grid, or I could do it with geometric contours?
I think Age of Empires and Starcraft 1 and Warcraft 1,2 were grids. Warcraft 3 , Age 3, Starcraft 2 are 3D terrain. 3D would be nice, but it is not paramount, if it is too much for Java to handle. I'd prefer to be 2D to save those headaches, then. I'm more interested in the gaming than the production. But a Martian combat 3D landscape might look good. ( "See you at the party, Richter" - Quaid, Total Recall)