so, I've got a lot of ideas. 6, to be clear. lets start out with...

-The Tank Tanker-

this is 2 smaller sentrys so you can cover 2 flank routes at once! you can only heal one at a time though, so you can only save one in a bad situation. next is...

-The Excellent Artillery-

this would upgrade just like the regular sentry, but a level 1 sentry would have a pipe launcher, the level 2 would add a flamethrower and burn enemies at close range, and a level 3 sentry would give it the ability to airblast only to extinguish burning teammates. after that, there's the...

-The Helpful Heal-

this would be a dispenser that dispenses 250% the health, but 0.50% the ammo and metal.

-The Fast Frontline Fueler-

this is also a dispenser, but it dispenses 200% the ammo, 150% the metal, but, 25% the healing.

-The Speed Pad-

this is a treadmill that if you run over it, it gives you a speed boost. a level 1 will give 4 seconds of boost, a level 2 give 6 seconds of boost, and a level 3 gives 8 seconds of boost.

-The Portal Gun-

this is my favorite concept. it would replace the primary weapon, and it would be the portal gun from portal. theres so much you can do! it can be a much quicker teleporter, and make it 2 way! you could put your sentry facing a corner, put a first portal in front of it, and when your in trouble, you put the second portal on the wall and have the sentry shoot out, do the same with the dispenser, etc! but it has 3 downsides. one, it does no damage itself, 2, no regular teleporters can be placed, and 3, your sentry and dispenser both can be upgraded to only level 2.