I don't happen to think this is a controversial idea at all. In fact, as you say, I think if there was a clear plan laid out which talked to improving / expanding the departure lounge and facilities, upgrading ILS, upgrading security scanners, recruiting a bigger ATC workforce and so on (there is probably much more) and said - this is going to cost £Xm over the next 3 years and as such we will levy £x onto each Departing pax......and look to generate £xm from car parking etc.
I think that would at least be an honest way of approaching it. It would prevent things being drop fed in (random parking charge order etc). Ultimately, the principle should be the passenger pays - in the short term that might needs more direct revenue through an additional departure tax - then on an ongoing basis annual costs are met through standard passenger charges passed through by the airline.