If turrets aren't meant to be deployed for long, maybe it might be interesting to rework them with that concept in mind? They don't have a lot of health, and now they only overcharge as soon as they're deployed.
Experimental Turrets does give incentive to keep them alive longer for their boons, but imo it's not that big of a buff (that, and the reflect is also only active for the first few seconds).
Otherwise, I like the concept of defending a point with turrets that would last long; but if I recall correctly, there was a bunker meta in PvP that was just unfun (which presumably led to the nerfs?).
I had the idea to give the turrets a lot more health, but it would gradually lose HP (e.g. 50k health with a degrade of 5k/sec so that it would last 10 seconds on its own; but this is just an example). They wouldn't last up to 5 minutes like they do now (sorry AFK farmers :-P). Enemies could still attack the turret so that it would go down faster, but it would still require some effort.
Since turrets wouldn't last that long on their own, a damage buff would be nice. It shouldn't be that big since we still have access to our weapons/kits; but hey, the option's there :-) (like the flame turret, I still internally cry when I see that 1 stack 3 sec burn on each attack).
But what I definitely would like to see expanded is their interaction with the tool-kit. The tool-kit now repairs turrets which by themselves don't do a lot of damage and are squishy by nature. It's usually better to pick-up/destroy the turret and blast the enemy with your other skills instead of hammering down on your mechanical buddy.
But what if the turret gets an additional upgrade when you start clonking on it? The tool-kit wouldn't repair it anymore, but instead it would receive more power/utility since you're not actively attacking enemies yourself (which would just switch the DPS/healing/cc from you to your turret).
Naturally, you wouldn't be able to hit enemies with your wrench when they're right next to your turret; otherwise it might be a bit OP in some scenarios ;-)
Also, you need to whack the turrets a few times before it activates the effect. An additional bar to visualise this would be helpful.
Just imagine this:
Your rifle turret would turn into a gattling gun with super rapid fire (I think I saw something like that in Drizzlewood Coast, makes me jealous ._. ).
Your flame turret would create a ring of fire that burns enemies around it. It would also give a fire field for allies to use (not constantly, but often).
Your healing turret would remove two conditions and apply additional healing with each upgraded pulse. It could also add a water field.
Your thumper turret would have a bigger range and apply additional weakness, but it wouldn't be able to this more than twice or it might be going a bit overboard (even though I love seeing enemies being flung in the air :D).
Your rocket turret would knock down the target area with each shot and cause additional bleeding.
The net turret is a bit difficult to be creative with, but the current overcharged version shoots an electric net. Maybe a thwonked net turret would create an electric field that dazes/stuns foes when the field is created? The electric field would last for a while so that you could use if for yourself (which is nice when playing Scrapper).
This gives the engineer more options for DPS/support, and the turrets have a limited lifespan so that they wouldn't quickly turn OP. Most of the effects wouldn't activate more than three times for example.
There might be issues going on with this idea and other stuff might need to be reworked too, but I mainly wrote this for fun (and to make turrets great again :v ). You might say this is my Wintersday wish list :-)