perhaps changing their role within the game could accomplish more or as said before a dragon point system on the map where each dragons have a certain amount of points
ie world holds 200 dragon points to be used from your dragon lair.
Green Dragons are 20 points red are 50 and royals are 100 with other rare dragons from events and such being put somewhere within those general numbers. this would keep the bigger dragons on a small leash if you will while letting the much smaller ones become alot more common to see around.
More of a compromise of sorts