Awesome... :)
I guess the next level would be lasers and missles. But then players are tempted to ditch the subtle skill of grenadering... I hadn't tried out the other grenade types yet but there should be ones that would explode on impact if there isn't. Mines would be fun as hell too. The upgrade aspect of the game can be geared to a diverging skill tree kinda like bubble tanks. Players may opt for fast weapon, grenade specialty, air-to-air, or just be plain speedy and small for a few suggestions. There should be a different badasslook for specific components for each specialty so i suggest getting out a sketch pad. Special components like teleportation, chainsaw and other fun tidbits is a good idea too.