This site doesn't support unofficial expansions or variants. That's fine, but how about having a Mission 0: a completely open hand with no tasks or win condition. Players could use it to implement any fan variant or mission that they want.
Eg Mission: any player must win all the cards of one color. So the players play all the way through the hand using standard rules. The site has no idea if they have won or lost, it's up to the players to use the log to see if they were successful. If, halfway through, they can see that they have failed, they quickly play it out and try again.
Of course, tasks can also be simulated. It could also be used to teach the game or try out different tactics.
Eg Mission: any player must win all the cards of one color. So the players play all the way through the hand using standard rules. The site has no idea if they have won or lost, it's up to the players to use the log to see if they were successful. If, halfway through, they can see that they have failed, they quickly play it out and try again.
Of course, tasks can also be simulated. It could also be used to teach the game or try out different tactics.