The wrench kit is underwhelming in pretty much every regards. It is supposed to create the TF2 engineer style of play, but it simply fails to deliver due to high cooldown on non-scaling turrets, and because the heal is pathetic.
So why not make wrench a bit more of a utility kit? An engineer should be at his creative best with his creative tool around.
- Make the wrench lowers cooldown for each tools (including turrets, elixirs, as well as toolbelts) by x second for every single action you use (can be improved to reduce more cooldown, etc, by traits).*
Taking into account that your swing animation is still the slowest of all weapons, it still wouldn’t be overpowered by any means.