So we all know that FSC is the #1 grinding mission throughout the whole game of SK, right? Well, how about once the mission has a check mark, you can only do that boss mission 1 time. If you want to do another boss run with the same boss, peoples should be directed to the arcade. All the newbs will start spending time going into the arcade. So everyday, once you do a boss run, you cannot do it again for 24 hours.
In reality, if Vanaduke was defeated, he wouldn't re-appear, now would he? So the main point is that once the boss mission is done for the day, you must wait until the day ends (12:00 = Everybody can do Vana by the mission again). This may be the same thing as a danger mission / prestige, but this is to revive the arcade and get more people to do it.
If we add this, it would make sense that once Vana is defeated in the mission itself, he won't appear again, but only in the arcade.
NOTE: You can help your friend do a boss mission if they invite you to the lobby.
-1
I can understand wanting to stop vana grinding, but your proposal is just doing the OPPOSITE, making it impossibly tedious (and costly) to get tokens or crowns.
How about once per day instead of once EVER?