Capabilities of the MuseScore Player

Realistically, I suspect the answer will turn out to be that it will contain that which is sufficiently easy to implement in a given amount of time - meaning it will be limited at first, with updates adding more features. With that in mind, I think that instead of just listing desired features, some sort of prioritization might be more helpful.

I'd say that the starting point would be what is on musescore.com: display of the score, animated playback., That seems obvious enough. Above and beyond that, I think the next most useful features would be change of tempo and transposition. Change of tempo I'd expect to work as a scaling factor as in the Play panel (so tempo changes within the piece are still honored in relative terms). Transposition I'd expect to produce reasonable results in simple cases, a hopeless mess in the more complex cases, just as is the case with the actual transposition feature in MuseScore (or any other notation program), and I wouldn't expect or even really want to see a ton of work go into making the complex cases better - there are others things I'd prioritize more highly. The MuseScore Christmas tune app released last year already shows that tempo and basic transposition is doable and basically done, so I'd consider that a given as well.

Being able to extract parts from a score sounds enticing, especially with linked parts (much less useful in 1.2, since the formatting of the parts won't be particularly nice). Lower priority overall, I would imagine, even though I personally would use this far more than I used tempo change or transposition.

Other features I'd rank as nice but somewhat below this would be playback features relating to practicing/rehearsal - having a metronome with count in, being able to loop sections, etc. But I could see others valuing this above anything else on the list.

For me and other jazz musicians, tying into some facility that can do some sort of chordname playback would be quite useful. Bob Keller's Impro-Visor is the one open source program out there I know that does this, but I have no idea if any of that has been ported to iOS or how feasible it would be to integrate the two. "IReal b" is probably the most popular commercial iOS app (at least in the jazz world) for this sort of thing.

Random cool feature I'd like to have for no particular reason except it seems fun: the Mixer, including ability to change sounds staff by staff.