Thanks, I hope to hear from you some more when there is a game to play!

I have made a system with rolling decoration that follows the player. It seems to work without slowing things down but my computer is really fast so I don't know. It's a couple of simple models of stones and plants that are randomly placed within sight of the player. If you move to far from one of these, it moves to a new location. The downside is that it doesn't work with the zoom or if the player would move really fast.
I'm getting ready to put up a webstart version... still very early gameplay but in my opinion an enjoyable game. Got some issues with game balance to figure out and some other features. Hope to have them sorted this weekend, but you never know.
Remaining is: Real particles instead of my colored spheres, Collisions, more enemies, a mission structure, menu system, pathfinding for the AI (once the tanks can't drive through each other and the buildings).
I choose a rather different gameplay for a tank game. I am inspired by MMORPG and GTA. You drive around on a big map and there are enemies spread out everywhere. Some places have tougher enemies. You must work your way up with upgrades before you are ready to play with the really nasty boys. This works with spawnpoints that have rules for how and when to spawn what enemies.
There should also be missions. When you see a blue ! on the map, you see an available mission. By driving into it the mission starts. Missions are driven by scripts, to anything can happen. They should often be micro stories that tie into the greater story. Mission give bigger rewards than just driving around and killing things. Some missions unlock other missions so you have to do some things in order (this is like in GTA). But like for instance wow, there will always be plain enemies everywhere to fight against. All missions can be played any number of times.
I have a plan that maybe a finished mission can turn of spawnpoints. So step by step, you clear the map of enemies, until they only have their fortress left.
I hope to have this ready very soon....