I think Orpheon could be a fantastic competitive stage, but it need a few tweaks.
First and foremost, remove the transformation. The second form doesn't add much - Halberd and Yoshi's Island already posses most of its elements. If anything, it detracts from the stage by diluting the unique first form. More importantly, the transition is incredibly problematic.
Sure, there's a warning, but even when it goes off I've no idea what's about to happen: the flip is just too unpredictable. There's no feasible strategy to respond to the siren mid-match and receive consistent results, which is totally unacceptable when those results could be the loss of an
entire stock. Possibly multiple times per match.
What's more, it's not something a player can use advantageously. Sure, one can try to keep the opponent grounded, but they have no clue what will come of that, nor even a guarantee that it'll be worse than if they hadn't. All one can really do is hope, which is a fairly noncompetitive technique.
In short, it's a stage that intrudes upon the match, forcing a response of which there is no "right" answer yet for which failure is fatal, and which facilitates little player-player interaction or even player-stage interaction. It deeply touches upon many attributes that consign to bannage, and even brushes against polarization in how well characters can handle the flip. There's not even a canon justification for it; it's just some dumb idea Sakurai poured Metroid-paint over. Overall, it should be banned while it has the flip, but it should not have the flip.
So, for the first and only form:
Less frequent platform movement, so each elevation has a chance to show its stuff, pausing longer on the less questionable ones. Shift the left platform inward a bit, and perhaps shrink it. Maybe something to catch ledgedashes on the right platform, or even add a whole underside if sharking's a problem.
This last tweak is less definitive, but I still think beneficial. I think the platform rising above the floor should return, and in fact should be expanded upon. Observe:
[collapse="Proposed Positions"]
Simplified, but the ideas should be clear. I didn't care to illustrate the underside.
Current:
Additional:
I am especially intrigued by the possibilities the first two "Additional" present; in particular, if the first could even
be tamed. (Perhaps if it existed only momentarily as the platform travels, rather than being a distinct position?) More could be done to make some of these less jank-prone if it's yet possible to change mid-match whether edges are hangable or platforms are fall-through.
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The fact that they are only temporary and effect only half of the stage should make what would otherwise be broken layouts manageable, and allow them to demonstrate their unique effects on strategy. Some may need to be nixed if they can't be comfortably available for any appreciable time, but we can't be sure without testing. And even with the platform's current positions the stage still presents a fair handful of interesting elements (I'd really like to see if its wall works out) and would be a welcome Counterpick, just not one living up to its potential imo.