This question is framed awfully for a number of reasons:
1) You're including in the premise that an alteration to the legality of Stadium should be made before any other stage
2) You're including in the premise that banning is the only option presently available to us
3) You're not actually asking the core question you're trying to ask, which I can only assume is because you have an agenda (banning Stadium)
Personally, I don't think Stadium is presently the most problematic stage of those presently legal, but I'm not going to get into that (and the stage I have a problem with is a stage other than Stadium and FD); you shouldn't be dictating the frame of discussion in this way as if it's the only option, you should be asking "What, if any, changes should be made to the current stage list?".
Similarly, banning isn't our only option - simple mods have become vastly more accessible to the average player now than they were 16 years ago, and it's worth asking if we should consider making slight modifications to the stages - or rather, if we should be using these versions of the stages in tournament (e.g. Frozen versions of Stadium, Yoshi's Story, and Dreamland).
In my mind, transformations / shy guys / wispy all detract from the stages they are present on, and our next step forward with regards to stages should be to make the current and, for all intents and purposes, sufficient stages that we presently play on more geared towards being fair, before we think about adding or removing more stages (if we, as a community, decide this type of modding is the way forward).
If we don't go down the modding route, then I firmly believe we should keep Stadium. I don't believe there is sufficient reason to change its legality, however there is more than sufficient reason for keeping it, primarily that it is the only stage with platforms that does not have a very high or "top" platform, which makes an incredibly important difference when compared to FD and the other stages in some matchups.