This is a topic I don't hear anyone talk about, not even top players, but these are some super glaring issues with Arenas that I think need to be addressed badly, cause it's becoming a genuine issue now, and patches don't really seem to be doing a single thing about them, probably due to how few people are talking about them.
1. The Arena Rotation System is too all-or-nothing. The issue with this isn't noticeable when there's only 2 or 3 players in the arena, but 4 or more you're likely to encounter this. Currently, there are only 2 rotation modes you can pick: Bottom One Leaves and Top One Leaves, but unfortunately, it leads to one of two situations depending on which you set it to. Bottom One Leaves, which is far more common in arenas, very often leads to one single person completely dominating the others in the arena, which basically means it's not only the same opponent every time, but one person gets to play far more. On the other hand, Top One Leaves leads to a similar scenario, albeit less likely, that someone will get absolutely destroyed and really just end up the arena's punching bag. Both of these scenarios have the same core problems: One person gets the most playtime, while the others are just forced to watch for 10+minutes usually, which is quite unfair. The fact that it's only one or the other makes it feel really hostile to uneven playing fields, and in some cases, even discouraging Arenas as genuine practice cause you don't want to deal with the broken ranking system.
2. The Ques are too Stingy. This also is a bit harder to notice when you have only 2 or 3 people, but it can become VERY apparent when the arenas have 4 or more players in the arena. When in the que, if you decide to change characters, stages, or music, or just so happen to try and check the rules, you'll be immediately kicked out of the que and sent to the back, giving the person behind you your spot. I could sorta get why this is in place, it just so happens to be poorly thought out from a developer point-of-view. Why can't it be that when your turn comes up that that's when you pick the character and stage (With a time limit of course to deter AFK/stalling), or why do you have to be punished for simply wanting to change to an alt? All of it ultimately makes us ask one question: Why make large arenas for friend gatherings online far less appealing?
3. Speaking of Rules... This one ties into the previous one, but why is it that you can't check for items BEFORE you enter the arena? Some people use Arenas for competitive practice, so why isn't there an option before entering the arena to see if options are turned on? You have to already be in there to be able to check them, and it can also lead to artificially clogging arenas since some will opt out the moment they see items on, but it won't show up like that to other players.
There's also an issue when it comes to bugs in the arena, but I have yet to personally encounter them so I can't verify those. Regardless, please Nintendo, fix this crap, Online is already really bad as is, and if the devs really care about the casual crowd as much as they say they do, they'll stop ignoring these issues already.