I was thinking that the Treasure Dragon could be a "possible" breed able, albeit a very rare one. It would give players (high-level players, especially) something to strive for. However, you could still only have one Habitat, so players that already had the Treasure Dragon wouldn't be able to hatch it (they could breed it, but not hatch it with a Habitat to put it in) and you wouldn't be able to have more than one.
As for what it would be breeded from:
Extremely rare by breeding dragons that collectively possess the colours Diamond, Yellow and Mythic.
With these colors, only the Gold Dragon possesses 2 of these colours (Diamond/Yellow) and the only Mythic Dragons are Mythic, Helios, Medusa and Atlas. That makes four, not including the Gold which is a necessity, possible Dragons to breed the Treasure Dragon from.
The Gold Dragon is an Ultra Rare and the Mythics are all at least Rare, making it very hard to get the Dragons to breed the Treasure Dragon and even harder to breed a Treasure Dragon from the two of them.
I think this would solve the problem of people not having the Gold to afford the Dragon while also still making it challenging to get. On the subject of people who have already brought the Treasure Dragon (or for anyone who breeds it and doesn't want it/can't hatch it), getting the egg as a possible breedable would offer you the chance to "sell" it, like a normal egg does, for a substantial amount of Gold say 500-1000. Maybe even the 800 Gold it cost to buy it naturally.
The above suggestion draws from MonkeyMomma76's "Golden Egg" suggestion
here.