Dragons aren't innately fire focused save for Magma Dragons though (Forgeworld model), which I believe does have some form of ward save against Fire.
High Toughness units would be interesting - anyone who runs Warsphinxes has a good idea of how that game plays. Deal of a thousand cuts is upsetting indeed, but it's fun to lob it against a unit of White Lions and watch as the High Elf go-to unit actually begins to struggle a little. I think I mathhammered once that a Fire breathing Sphinx charging a White Lion unit actually wins combat, just about.
Really though, the main problem people have always had with Monstrous Mounts is that they're often too expensive and the character riding them was also vulnerable to cannon sniping, as well as being locked down in a challenge. If I had the option to take a riderless Dragon I'd be spamming them all day.
