The Ability to save a duel and pick up where you left off another time/day/whenever you feel like it

You create a lobby and start dueling your friend(s) then one/both of you have to go for whatever reason.

The duel gets automatically saved during each turn in your/both of your device(s)

Next time you and your friend(s) are free you can load the duel using that saved duel file

After you load the duel it brings you to the hosted lobby screen with the player slots, now all you have to do is have them join your lobby in the same player slot they used to be in when you started the duel

If you want you could also let any player (not just yourself and your friend(s)) join the loaded duel hosted lobby in any slot including yours and your friend(s) which would then put that said player in your friend’s stead with his hand/deck and lp and everything else your friend had prior to stopping the duel

You could then use that to either:
-create challenge situations and test people (think of it like an online duel challenge)
-try different scenarios starting from the same turn
-have someone else fill your friend shoes

You could also go further and save every turn of a duel and store it in the saved duel file, that way when you want to load a duel using said file, you can also select which turn you want to load and start from

If you think allowing anyone to join the saved duel would cause security issues then only allow it in private lobbies and if all players inside it agree to save the duel (with a prompt) then turn it on, if not then that duel wont be saved.

I got the idea from the Civilization games where u can do exactly that.