As far as the current airline lessors, the commonality penalties have been pretty much eliminated, since only scheduled aircraft is counted.
As far as a player starting out as a leasing company, that might lead to some problems, since the leasing company needs extra capital, and if a player starts as a leasing company, then he would funnel this extra capital to his airline for faster initial growth.
So probably, the order should probably continue to be that the player needs to earn the capital to become a leasing company by being an airline.
As far as setting up a leasing company that would be eligible to bid, to become a lessor of new aircraft players order, that is a fantastic idea. I think there needs to be some sort of check on the capital availability for leasing of the new aircraft. It should not just come from thin air, as it currently does, where the AI lessors have unlimited credit lines. Both AI and player lessors should all be subject to credit availability. As the demand for aircraft explodes, so should the interest rates and leasing rates.
The manufacturer may have a price of their aircraft, but for aircraft to be purchased, capital has to be obtained. For example, if a player goes to order an aircraft, all the lessors will be willing to bid on that. But all the lessor could be completely tapped out, there are no bids from leasing companies, and a player could just get an answer that lessor could not be found. So the player would have to lease a used aircraft from the stock of the leasing companies, which would allow them to build up capital.
As far as lessors, they could have standing bids (so that the leasing for an airline would be instantaneous). For example, a leasing company can have a standing bid to finance 500m worth of aircraft in 767 and A320 fleet types, and the leasing company would be able to set the leasing rate - monthly lease rate as a fraction of the manufacturer price of the aircraft. And further, the leasing company can set different rates based on credit rating of the lessor. The system would go through the leasing companies, and get aircraft from lowest rate to highest rate, with order possibly being split between several lessors.
The leasing companies should be able to use their down payments on aircraft as security for loans immediately, and their credit limit should be substantially expanded.
The AI leasing companies should be subject to market forces, and eventually (hopefully) all will go out of business, and only player leasing companies will remain. When a leasing company (AI or player) goes out of business, all their aircraft should be dumped on the UM as Purchase Only, at substantial discount, so that other leasing companies that are healthy (or airlines) can get this aircraft at deep discount, and in turn become healthier / more profitable. No longer would the leases be automatically be taken over AI lessors with money pulled out of thin air.
All lessors being subject to credit availability will finally put the breaks on the exponential growth of airlines early in the game. As the orders for new aircraft start to come in at a frantic rate early in the game, the demand for credit will go up (so should the interest rates) and in turn, the leasing rates will go up - which will lower profitability of airlines, slowing them down. Or the airlines will have to slow down their growth, accumulate their own capital for aircraft purchases.
The Beginners World could have AI lessors with unlimited credit, so that airlines there can just keep ordering (Maybe the Holding company concept could be hidden from the Beginners World, with the UI being either "Normal" or "Advanced", with Advanced being available only in full game worlds. But the full game worlds should have constraints on capital availability, which can slow down these worlds to a crawl, until capital is earned (flying pax, profits from leasing) and reinvested into more aircraft.