Upgrade: Slow field
Quick summary
An upgrade that will create a visible area around the player which slows enemies that pass through it.
Why it would be valuable
Instead of being just a plain offensive or defensive upgrade, this potentially has a lot of tacticle applications that will probably only be discovered once it has been made.
Proposal
When a player buys this upgrade a coloured circle of some arbitrary size would appear, centered on the player. This circle (field) would move with the player. The circle would be coloured the players team colour so that people could easily be aware of its range. On the screen of the player that purchased this upgrade the edges of the circle could be blurred to a greater radius that is determined by the smoothing time of their client. This would make it more clear that it would work inconsistently in that area. Enemy players in this zone could have their momentum halved for the duration that they are in it. Bullets and other projectiles could also be effected. The upgrade should probably only last for a fixed amount of time.