My 2 cents.
It's been mentioned before that the used a/c market limit works well. So, why not implement these limits to slot purchase (also mentioned before)?
Each base airline receives say 10 weekly 7 day slots as part of their 'buy in' with the airport authority. After say a month, they receive an additional 1, 7 day slot allocation from the airport every week ( or two ) . These can be banked so as not to harm casual users, but expire after maybe 3 months. Noone flying into each airport is able to open more than one flight per week either, so as not to disadvantage the based airlines too much. Trying to purchase slots in addition to your allocation will result in the message 'the airport planning deaprtment are still dealing with your previous order'.
Also, how we grade airport sizes needs looking at urgently. The recent case of EGPF, Glasgow has been raised. It is classed as a 'Large' airport. In 2012 it handled 7.1m pax. KIAH, Houston GB is also a 'Large' airport, in 2011 it handled 40m pax ( source wiki ).
Regional airlines seem to have been stung badly here, just after all the work to reduce the small a/c penalty has been completed...