Endecagram (11 lines staff)
I'm sorry I don't know if it has got a name in English. In Italian it's called endecagramma (11 lines staff), opposed to pentagramma (5 lines staff).
As a matter of fact, an endecagram just has two 5 lines staves, like in a normal piano staff, but there are three differences:
1) central C is always drawn in the middle of the space between the two staves (where there would be an 11th imaginary line, hence the name);
2) there's a single voice running through it (i.e. no pauses on one 5-lines staff even if all the notes are on the other 5-lines staff; no use of ledger lines between the two staves except for the central C because the melody can go on both staves as needed);
3) usually there's just a C clef on the central imaginary line, but you can also find the C clef together with treble and bass clefs or just the treble and bass clefs as in a normal piano score.
You can see some examples here:
Note that on MuseScore you can't simulate an endecagram with a piano score, even if you are ok with having just the treble and bass clefs, for two reasons:
1) you can't get the central C drawn in the middle of the space between the two staves (at least unless reducing the space)
2) you would have to switch between right hand and left hand continuously while inserting notes, then to make all the pauses invisible (and the first step can be very time-consuming and frustrating :) )
Endecagrams are widely used at least in Italian classical solfège courses. As a teacher, I've already been in the case of needing it. :)
Maybe it could be added as a new "solfège" instrument?