Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
There is already a way to do so. Create a groupchat with the/makegroupchat command, create a tournament e.g. /tour new gen8ou, elim, and set the rules e.g. /tour rules !Endless Battle Clause, !HP Percentage Mod, !Sleep Clause Mod (the ! removes a clause).
To add a rule, simply write the clause without the !.
Then invite the user you want to challenge and both join the tour. Use /tour start. Here you can find how to adjust other rules. A list with all the clauses is at the bottom of the linked post.
I'm not sure if I'd like to consider this done without some sort of UI support. Zarel do you have anything planned on this front for the preact client?
I'm not sure if I'd like to consider this done without some sort of UI support. Zarel do you have anything planned on this front for the preact client?