I mean, I don't really know any other game where promos have become part of the regular game in this manner. I know nothing about Magic, does it even have promos?
In the early days there was a card you only got at one con, that they then included in an issue of the Duelist; and there were cards that came with books, which were never available other ways, which included one very powerful card, Mana Crypt. These cards are all legal in the format that lets you play with any cards. I think they recognized that this sucked and since then promos have just been normal cards from expansions, either the same as in the expansion, or with alternate art, or foil, or both of those, or giant-sized, possibly with alternate art.
I do not have much experience here, but Dominion, Kingdom Builder, and Gauntlet of Fools all have promos, and that's three different publishers.
It's also weird because some of the promos were just regular cards that were taken out because they wanted a promo.
Zero of them are this. Four of them are cards that were taken out for other reasons and then that made them available to be promos (Governor is the exception and was just designed to tie in with Puerto Rico).
So I'm not really sure where I stand: Should we just embrace the promos as part of the Dominion canon or should we treat them as variants? If someone has compelling arguments either way I'm interested.
We are talking about Goko.
On Goko, promos are hard to get; currently you need to beat several campaigns and then choose to "buy" one over the other promos. Black Market and Stash aren't even programmed yet but of course they will be.
Once you get the promo though, that's it, it will randomly show up in games you start just as often as any other card you got. The only thing that distinguishes it is that most people won't have it.
For sure you do not want extra setup decisions to spring into being if you have Black Market show up in a game you start. To do such a thing for Pro games you would need to pick a player to get to make that decision, which would not be great. For casual games you could have the decision made in an options panel somewhere, and just stay whatever you picked last for future games that you create that have the card.
But whatever; here is something to spend zero energy on, instead doing things that will make the game better, like making the unplayable campaigns playable. The FAQ's "for example, you could agree to use one of every Kingdom card you own that is not a part of the Supply" looks good, let's go with that, problem solved, only a little time wasted. Let's put "hey the Black Market deck could maybe be generated some cool way, whether or not it's some way that will suck the joy out of it for people who actually love the card" way way way down on the list of things to pretend to someday get around to.