SEE THE 3D MODEL: https://skfb.ly/BKPC
9314 Tris
Texture Red/Blu 2048x2048

Third person remotely controlled Sentry Gun with the wrangler. Left click to fire where you're pointing, right click to move it like a strategy game. You must run beside if you want to repair it, remove sappers and be able to see enemies. That mean the more you are vulnerable the easier it is to control it.
Similar gameplay: http://youtu.be/srmDpNNIrUQ

Just a stat idea (Valve only can choose the stats):

Role:
Good in large and open areas, good to support behind a pushing team.
Bad in front line (if sapped), bad in close areas (no splash to fire behind a wall, can't jump and a fall deal critical damages).
Can't defend points/intelligences (no auto aim or slow rotation speed, no shield protection).
To understand how the sentry works this video is the closest: http://www.youtube.com/watch?v=srmDpNNIrUQ

Need a new PDA or wrench to use it (see the collection "Mobile Sentry Gun pack").

+can move with the wrangler (right click where you are pointing like a strategy game) even underwater. Continue to move at the point you have set when changing weapon. That allow you to defend yourself instead of carrying the sentry.
+can be set to follow the engineer without the wrangler.
+HP between level 2 and 3 = 175
+explode when destroyed and deal damages (not sure)

-if the sentry has auto aim (not sure) 30% turn speed compared to level 1 sentry when auto aim is activated, can auto aim only if the sentry isnt moving or controled.
-no shield
-less DPS than wrangled mini sentry / less DPS than heavy in close range, more in long
-cost 200
-not upgradable
-not grabbable, can't take TP with it
-you hear it when it moves

50% interaction nerf for some shotguns
-Rescue ranger heal 25 HP instead of 50
-Kills only grant 1 crit per kill for the frontier justice instead of 2

-where the sentry is not allowed to be placed, there is an invisible wall for the sentry, for example to get into the respawn

Optionnal:
-requires metal to move
-can't refill the ammunitions while moving

Suggestion by Raptorian:

Assuming this robot is tied to a new wrench (or even a new PDA), the new wrench grants a new mouse2/ mouse3 function; paint waypoint.

The robot will do its best to move to the last waypoint assigned to it, and the Wrangler will only take command of its turret while the robot can trundle along for strafing actions. Standard Wrangler buffs/ shields/ turret reboot times apply, but the robot can still move while the turret is rebooting or being overriden by the Wrangler.

You can assign a new waypoint while wrangling the robot's turret.

Should the Engy that built this thing died, it'd move to its last waypoint and then sit there gunning everything down (or being very vulnerable to everything, it depends of the waypoint). If it was being wrangled at the time it'll probably die (sitting still, rebooting its main gun).