So how about making the free dragon play limited to once per turn by turning it into an activated ability
"
Lair: Play a dragon for free. It enters delayed 5 turns.
If Cave is destroyed all dragons take 5 damage."
Fractal and chimera are still ugly, but you only get to play 1 dragon a turn per cave, so you will have to build over several turns.
Granted, you could use several caves, but that takes a lot more time and coordination to set up.
It might still be OP, but its a definite improvement I think.