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Melee mode: Bullets remain stationary, and die instantaneously, so you have to actually get your gun inside of the enemy tank to kill it. Kind of like SR, I guess, but you still have to aim and trigger the death manually; it won't just kill the other guy automatically if he's within a certain radius. Maybe a flag could be set to extend the bullet's lifespan for a much longer period of time, so that you can use them to set traps. Maybe set a limit to how many traps can be set at a time (say, you set three bullets and then after you set a fourth one, the first bullet dies).
Antigravity: TZBZ said he tried something like this before years ago. Everyone spawns with WG, with a practically endless number of flaps (like on HNSH). Gravity would be set to a positive number so that all tanks will naturally rise, while WG Jump Velocity would be set to a negative number so that it can counteract the gravity. Basically, jumping and falling are reversed. The variables would have to be set so that it's easy to control your tank. This could work really well on a map with no floor, with death zones at the top and bottom so you have to prevent yourself from falling or floating too far.
GM modification: Not trying to replace GM or make another flag idea, but as something to try out on a server to make GM feel like a different sort of weapon. Instead of being a "fire and forget" sort of weapon, this would give you only one GM that you have to guide towards the enemy, so you have to invest some attention into the shot. The GM would move failrly slowly, but have a long lifespan, good turning, and maybe a larger size. You would still control it by locking onto things, but you might have to maneuver it around obstacles to make it reach the target, which you would have to do by changing your lock since I highly doubt BZFlag will ever allow direct control over GMs (and I'm not asking for that). Might be difficult to pull off in practice, but maybe you could use your teammate's positions to help you aim. Also, since you have to pay more attention to the path of a shot, this could leave you vulnerable to attack from other players. As a point of comparison, think of this as being sort of like Snake's side-B missile move in Super Smash Bros. Brawl, if any of you are familiar with that. If not, go to 0:58 here to see what I'm talking about: https://www.youtube.com/watch?v=AV-LKZdhsVc
I think that's about it. Love 'em? hate 'em? Let me know what you think.