How about make dragon size, health, damage and behaviour (for example sometimes they wouldn't land at all) random? That way some dragons could be easy and then there could be huge dragons that would be almost impossible to kill, good for both hardcoe players and casual players. Make it so that size still matters big dragon always has more health and damage than little dragon and race should still have effect. I remember a mod in Oblivion which made it so that there can be different health and size enemies, so it definitely could be modded.

I'd really like that. I am playing on Expert difficulty and I can kill dragons so easily , they can't even remove half of my health bar.

Sounds good. With upper and lower limits for the diff types of dragons. Also be cool to give them some random spells instead of just their "beam".