Oh, and, I’ve another concept that builds on top of your suggestion. It’s one that’s been knocking around in my head for a while without a game to bolt it to: about how to encourage people to create this content other than for the fun of it.
When a user creates a challenge, they can set it up with a prize in gold, and a total prize pot. Each attempt costs 5g, the first successful attempt per person wins the prize (up to the prize pot limit to the creator isn’t bankrupted). The entry fee is paid into the creator’s account, and the prize taken from it.
This means that if you’re creating a challenge, you want to create one that will take many attempts before someone figures it out. If you’re confident your challenge is hard, a high prize might attract more people to attempt it. You might set your prize pot to allow 3 or 4 winners before the prize is exhausted and the puzzle is withdrawn. The ratio between the prize and the remaining pot controls how many people can attempt it at once. The ratio between the 5g entry fee and the prize establishes a “par” for how many attempts you think it should take someone.
Challenges and attempts should be limited by cards available in people’s collections - so if your puzzle requires the friendly general to have a Dark Seed and a Sphynx and the enemy general to have an Eclipse, then you’re only allowed to attempt if your collection contains a Dark Seed and a Sphynx, and you’re only allowed to create that challenge if YOUR collection includes a Dark Seed and a Sphynx and an Eclipse. That keeps the element of progression.
Challenges will also need the normal turn timer, so you can’t just stare at it all day to solve in one attempt.
And you add a challenge search facility, that shows currently active challenges, and allows you to search / sort by the prize and other stats.
Of course, it would be possible to cheat a bit, to solve it and then give the solution to your friends, to publish solutions and screenshot boards. But if it’s of the order of 5 gold as per the old challenges, then the incentive to cheat for profit vs have fun is reasonably low.