It wouldn’t be so much a tower as an ability to help towers. Terraforming where you can put water on land and land on water such as was in the last game.
A tower though… ummm…
What about a Specialist Monkey?
It would start off like a spy, able to see camo, uses a silenced pistol.
The upgrades at first would be a mix between Ninja’s and Snipers, one path leading to the Spy, the other to the Fire Support towers.
The upgrades (I can’t really think of prices sorry) would be like this.
Increased vision (Range)
Flashbang, throws a flash bomb close to the monkey that will pop a layer (Not lead or frozen) and slow units massively.
MiB Training, giving 2 automatic silenced pistols and allows surrounding bloons to see camo bloons.
Saboteur, Instead of slowing down bloons, they all come at half strength for ten seconds.
Increased Rate of Fire
Increased Caliber, only pops one layer but will pop lead
Pathfinder, increases range to full map and looks more like a sniper monkey laying on the ground with a gilly suit on.
Fire Support, This will be the most expensive upgrade but allows popping a full ceramic bloon as well as use of an ability called Fire Support which would call the aid of off screen mortars for 2 seconds firing in at a chosen point with enough to destroy an entire MOAB. This would take a long while to cool down though and he would not fire as fast as a sniper monkey’s semi-automatic only the faster firing upgrade.