Avahz Darkwood
Emperor
I think it would be a bit much to link it to all normal heroes.
Almost every civ will have a hero at some point, likely fairly early-mid. It would make the bonus kind of pointless when everyone has it.
the mega units are much rarer.
Well there arn't really that many heroes per civ, you could possible have more mega units. If you take you example below, then their will be way more level 12 vamps than the two or three heroes per empire (excluding the Grigory and a few lucky events). Either way the general suggestion is a good one IMHO because mega units are quite important... But as with everything else I am sure something like this one will not make it simply because of feature lock, even though its a good idea...