Different Key Signatures for Each Staff
Modern music notation sometimes requires the key signature to be different on each staff of the same system (I'm not talking about "transposing instruments").
Sibelius already has this functionality as you can see at the bottom of this post :
...or directly on this picture :
Also it would be very usefull to be able to do the same with time signatures, like 4/4 with 12/8, or 2/4 with 6/8, etc.