I personally have always disliked the exclusivity of unbreedable limited creatures. The older they are, the more they cost to buy from other users; some are seldomly seen for sale. Let past limited, unbreedable creatures become breedable.
Would it break the market? Maybe a little, but I think stream born, shop born, event born, and quest born creatures should remain the same asking price they are being sold at right now by fellow users. There has always been high value in creatures that are progenitors. The prices for their offspring would then be less depending on the generation. Basically the gold values would function the exact same as current breedable creatures.
Allowing unbreedable critters to breed would also make limited releases less stressful. I know I personally get a weird sense of urgency when I see "This creature is limited and will not be able to breed." And some of us can't be online enough times throughout the event to grab enough of each creature being released because of real life obligations. Especially around holidays like Easter, American Thanksgiving, the winter holidays, and New Year (feel free to include your country's observed holidays; I only listed US holidays because I live there lol) when many of us are prepping for these holidays or traveling to visit and be with family. At least you or other users can breed the creatures at a later date. It would help those of us who collect pairs of adults, hatchlings, and/or frozen eggs.
On top of that, there's also plenty of new users that miss out on said limited creatures. It can feel daunting looking at the list of all the creatures that can't breed. And with Gold Mining having to stop indefinitely, it could take ages to obtain them. It might also be difficult if no one is willing to sell you any of their progenitors. As I said before, doing away with unbreedable creatures makes collecting feel less grindy.
This would also make it more fun to build armies and hoard of our favorite creatures that currently can't breed. (Personally speaking, I have a small army of Giant Sylphs, Pocket Sylphs, and Ornate Sylphs--all unbreedable. If they could breed, I'd have an entire military force of Sylphs lol.
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"But what about the Black Market?" Still have it come to visit us, just as it always has! Alethin can still offer previous event eggs if some us want to collect progenitors instead. Nothing would change how the BM functions.