The OP is absolutely correct regarding the 15% penalty not being realistic. Not sure how difficult coding would be, but a more true to real life model might look like this:
You would have two different maintenance options and could freely choose either for each of your AC types at any given airport/maintenance location.
Option A: Outsourced Maintenance
1) only hourly+parts (or per maintenance block) charges
2) no 15% penalty
3) per aircraft costs are higher than in the following "in-house" model
4) this option will make sense for starting airlines and/or singe (or small) AC fleets
Option B: In-house Maintenance
1) the 15% penalty is non-existent and is replaced by each AC maintenance type having a relatively high fixed costs (to cover parts, type specific training,...). This "type overhead cost" would of course vary depending on the type. E.g. a B777 or A380 overhead would be significantly higher than a LET or ATR overhead cost.
2) individual per-aircraft maintenance fee then on top of the overhead fee
3) mechanics would be treated and paid just like your other staff... thus their salaries would affect their morale... thus affecting quality and effectiveness
4) this model will make sense for fleets with larger numbers of AC, and would be quite cost prohibitive for singe (or small) AC fleets (especially larger jets)
This then allows for further development of the maintenance model by adding features such as:
1) Maintenance contractors (for outsourced maintenance) are airport specific (smaller airports may have limited choices, varying prices,...)
2) Hangar/shop leasing - in-house maintenance would then be possible only at airports where you have leased space to do it
... 1&2 lead to a more realistic airline behavior (e.g. discouraging aircraft being left overnight at just any random airport for a "maintenance block". Instead have an incentive to set your flight plans such that they return to airports where maintenance is possible / or cheaper.
3) Being able sell/do maintenance for other airlines (if your capacity permits)
... Leading to a scenario where e.g. your parent holding company or one of your subsidiaries or airlines has one set of shops/employees/parts... and is able to service your fleets across all your subsidiaries.
4) Scheduling of annual/hourly/cycle inspections requiring AC to be pulled out of service for a period of time at regular intervals (necessitating the need for stand-by aircraft or pausing of service if none available)
5) Random mechanical problems requiring unscheduled maintenance resulting in flight delays. Their frequency/severity would be proportional to the age of aircraft and quality of scheduled maintenance.
(My apologies for the lengthy post, but I do believe the maintenance aspect of the game has a huge potential for improvement and added depth.)