It would be convenient with an option to place gradual tempo changes (e.g. rit. and accel.) in the middle of the grand staff.
This position seems to have been ubiquitous in the piano editions of the large publishers (including Breitkopf, Schott, Henle, Universal, Peters and Schirmer).
Of course, these tempo marks can be moved manually, but unfortunately, at the risk of colliding with other elements afterwards.