This one was a quickie.

Oh no, not yet another flying sentry? Whatever will we do?

The Surprise Sentry would allow the player to launch a small, low-powered flying sentry from a distance (similar to the distance of Demomans bombs). It would have a small health pool, and a short lifetime (10-15 seconds) before it self-destructs. During this time it will act like a mini-sentry.

The Sentry itself would remain static when deployed, it would not be controllable, repairable or movable. The fact that it can fly is purely a visual effect.

The Surprise Sentry replaces the primary weapon.

The Surprise Sentry would be used mostly for creating distractions and confusion, and an engineer can launch only one at any time. It could also be used to gauge what may be around a corner (Shoot it around said corner, if you hear shooting, you know what to expect).

These are just my own ideas though, feel free to discuss different ones.