umm way too complicated much? how about an egg that has a 25% chance each turn of hatching into a dragon of your mark? or has a cost to hatch into a dragon of your mark?
Good ideas, but they aren't exactly in a same page with the original idea I'm trying to accomplish here: I want to introduce a mechanism that makes some situational cards more usable, when they traditionally have had the potentiality of becoming "dead cards in hand". For example Rage Potion is a great boost/creature control. But if you use mono- 
deck without Lava G.s, that card becomes only creature control. This new "Dragon Egg mechanism" would now give new alternative use for that card.
or hatches into a dragon of your mark whenever targeted with a spell? or even when targeted with an effect of any kind turns into a dragon of that element?
That is pretty close what I've tried to do here. The only difference is that I came to a conclusion that if damage cards (Lightning, Rain of Fire, etc) would also give you "free Dragon" it would be OP; and like discussed earlier that would make Pandemonium combo way too powerful.
the version you have here would never be implemented
Yes, you're probably right. I've also seen the cards that have made to the game - Phoenix & Voodoo Doll. The original ideas for those cards weren't actually that close to the final cards, so I'm sure that this idea would also be adjusted and tweaked a lot if it ever goes that far. For now I'm just happy that this idea of mine has been noticed by some forum members, and hopefully I'm able to get this to Crucible soon. After that we'll see what happens.
I got yet another idea for solving the problems SnoWeb and Hyroen pointed out (inhomogeneous costs with different spells, and "too easy way" to get Dragon on turn 1):
I had originally idea that the ability was
passive (like in Mummy), but what if the passive ability would give this Dragon Egg
active Hatch ability (like in Fate Egg) when it gets targeted. The active ability wouldn't be usable on the turn when Dragon Egg was played, so you would be able to attack earliest on the next turn (problem Hyroen pointed). The activation cost that would be needed to actually get the Dragon hatched from the Egg could be adjusted according to the cost of the targeted spell/ability used (problem pointed by SnoWeb). I hope you get the idea, even if I couldn't write it down more understandable.
*edit *
I just posted this to Crucible Candidates list. *keeps fingers crossed*
