Just need to decide some basics:
Length of season.
4-6 weeks sounds good. I like shorter for this one as there's nothing stopping us from just starting another one as soon as this ends. Can just hand out prizes once a month and reset scores and have an ongoing league.
Maximum number of times you can face your opponent (if any)
Why would this be a thing
Minimum time until you can duel the same opponent (if any)
I vote for "one duel per daily card", so basically working out to once a day but without having to do any bookkeeping about waiting around because you dueled last night and want to duel again this afternoon.
What kind of point system should we use?
Don't care
Additional comment: The rule about how if you don't get a certain number of duels done in the season, you have to wear a giant scarlet letter on your chest and recite 150 rosaries and don't get to play next time is dumb. I don't see anyone suggesting we import it, but we shouldn't import it. If we need something similar, make a participation prize that only goes to people with enough in-season duels. Carrots > sticks.
edit: With regard to allowed upgrades, defaulting to unupped in the event of disagreement is optimal. Anyone can agree to fully upped or 6 ups or whatever, and it would make me happy if the de facto standard became 6 ups per duel (or even fully upped), but having the
fallback rules include ups allows wealthy assholes to effectively auto-win against poor people by just not agreeing to play unupped. Fallback/default rules should be greatest level of inclusive.