Well, you can always challenge a friend to a duel, so there's that.
But you can't change the rules of the duel as you mentioned, unfortunately.
I don't know if that would be implemented into the game at some point. I don't know if any card game ever has even implemented something like that xD
I would be interesting though, but I really doubt we will ever see something like that besides the daily challenges.
I know i can't change the rules of the duel... That's why i suggest it.
The daily challenges are really fun with random conditions, and missions, is just that, put thats in a multiplayer fighting 🙂 or more crazy conditions. It will be great for us , I do not think it's something that difficult or that it costs them much to do that.
Let's be creative:
Fun Modes:
Eureka: Every player draw +3 cards every turn ( 2 of the entirely game,and 1 of our deck), and can have 15 cards at most in hands.
Really a mess: Every plant and zombie draw one card of a class different of the own Hero classes. (For example if you are Green Shadow, you're plants maybe draw cards of solar, kabloom or guardian).
Don't waste time, just fight: The game starts in the round 10 (that mean, you receive 10 sun/brains in the round one).
Prepare you fight: Plants and zombies can only start attacking after turn 5, but you can use tricks before.
The world upside down: All common plants gets + 4/4 when played. Rare, super-rare and legendaries gets less half of its life and attack.
And yeah, the others conditions we all know in the missions:
every plant/zombie gets +1/1 each turn, end of the turn each plant/zombie with the more health does bonus attacks, end of the turn everything gets 1 of damage, etc...
