Unusual time signatures
One request that would be quite a low priority in the scheme of things would be to facilitate unusual time signatures, such as 5/6 or 3/10 or the like.
This would be something that I do not believe is possible in Finale or Sibelius without extensive kludging. (e.g. in Finale the t.s. would need to be created as a graphic and inserted manually into every staff).
Comments
With kludging it is also possible in MuseScore: http://musescore.org/en/node/3064#comment-7402