One solution might be to make it impossible to do 0.5 frequencies where a "1" is possible.
Ie. Heathrow to Dusseldorf with a 737-300 (No 0.5's - maybe 1.5/2.5?)
Heathrow to Johannesburg with a 767-300ER (0.5 frequency allowed, as thats all the aircraft can manage IIRC)
It'd maybe be more complicated, but could it be tied in with demand as well? Where routes with average or poor demand could be flown one way, but routes with good or excellent demand couldnt? (where range allows).
This might encourage airlines to operate "unpopular routes". I might send an ATR42 from Cardiff to the Isle of Man one way, but I might not be able to turn a decent profit doing a daily return.
in the real world, airlines often operate 3weekly frequencies on long haul or low demand... but rarely do for high density commuter routes. I see no reason why a longhaul flight cant land, stay overnight and return!!! I also dont see why a shorthaul flight would only fly one way when a return home is perfectly possible!
Apologies if this has been suggested before.