Engineer was the class I learned GW2 on and remains ones of my most-played, but...I rarely ever use/used turrets for many of the reasons stated above by many of you.
You know, one of the biggest issues I saw repeated was the lack of mobility. So, just...make them mobile. Have it where when you pick up a turret, there is no cooldown; instead, you can immediately drop it somewhere else (the exception being the healing turret and supply drop). Maybe give the pick up ability a half-second channel that can be interrupted, for balance's sake. If you detonate it or it gets destroyed, then it goes on it's full cooldown. Turrets slowly repair after being picked up.
The overcharge effect can occur when they're detonated rather than placed. Maybe rename it to overload and remove the explosion, and you get a field and a projectile effect, but your turret is sacrificed. Rifle turret drops a lightning field and fires a burst of several rounds, basically emptying it's clip and applying some vulnerability at the same time. Flame turret creates the obvious fire field and launches several projectiles that combo to inflict burning. These can target in an AOE around the turrets' location.
No bonuses when they're destroyed though: you shoulda paid attention. In exchange they need more defense so they aren't just casually one-shot. This is something else that can possibly be fixed by scaling the turrets off the engi's stats.
(Edited for spelling/grammar. Thanks autocorrect.)
