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I like this suggestion! Other matchmaking platforms (such as most chess sites) have this kind of feature.
I think only team preview formats should be eligible for this. Selecting a Pokemon would suffice to say "I am here," and if all parties do not do this, the game would be aborted.
I also...
hi im your opponent for random battles. I'm GMT -6 and i'll challenge you but likely not respond. We have polar timezones, so how does right before bed sound? i'll be fairly tired