From a fresh approach I suspect the real issue with 'Types' is the impact on the Maintenance Department.
Each new type requires all the necessary number of maintenance teams going off on training before the type is introduced, or employing already qualified maintenance crews.
How many staff needed per aircraft type, how many types do their certs cover, and how many can one team service on an individual aircraft type, doing all the A, B, C and D service checks in a year).
For each maintenance team there is:-
1a. a single set-up cost,
1b. plus an annual renewal cost,
1c. plus a limit to how many aircraft they can maintain on the same day and in a week in total.
2a. Then there's the spares required, a whole new warehouse full for the first of a new type.
2b. With a monthly cost to cover the parts used by the number of the type in use.
Note:- the Base Size should have a limit to how many types it can service as well as the size of aircraft because of the Parts and Staff Accommodation and Work Areas.
Now this would also require that the big fleet operators no longer service all their aircraft on the same day by reflecting the real world cost and issues in trying to do this. It's a huge change for the Sami team (of one and a bit?) but perhaps a halfway/partway step or fudge would help. A better model base for the number of staff in engineering for example.
It has a built in penalty for taking on a new type, training, parts etc and that cost will continue to penalise fleets of one or two of a type.
The current system should reflect that the airline has leased out the maintenance to the manufacturer. The first plane again being at a big cost (non-resident parts and team with occasional over-runs). Should any additional plane types from that manufacturer be at a lower cost.
Since the manufacturer is involved then ALL of it's range of aircraft can be covered under one agreement though the Parts would remain a specific type cost. ie 737 v 747 don't have much in common, nor do their engines. It means a big cost in moving to, or taking on, a new manufacturer which will help to limit the Types each airline operates.
This approach to Servicing aircraft could also apply to Engines and be applied in a similar way.
FUTURE or temporary measures:-
An option is for the Real World's "Wet Leasing" arrangements where the plane is staffed, fuelled and maintained by the owner, all at a charge of course, weekly for maintenance plus hours flown or fuel used for usage.
Ground crew is another area where the game seems to go for a fairly simple approach. Real World the number of ground crew determine how many aircraft can be turned around at the same time. Size affecting the number of Ground Crew required.
11Air - ElvenAir - in several games with sub 100 seaters but still learning how to use post 70's Boeing types.