Tried to script a stupid piano roll
Quite clever for something stupid? Perhaps it sucks for *actual* composing (well, haven't tried) but as a proof of concept it's pretty damn good.
Also, when you are saying that a piano roll would be a one-way street (from tracker --> roll), I guess that you are referring to a full-blown pianoroll with room for >120 notes - so, the limitation of 12 note columns? Because, otherwise I see no problem converting back and forth*. And to me, it would pretty essential that I didn't have to choose a specific track type in advance - one-way streets are a killer for the creative flow.
* Well, apart from - as previously talked about - the challenge of how to represent overlapping notes. Pianorolls suck for this kind of thing.
As for the talk about horizontal vs. vertical space, zooming and such... In my experience with pianorolls, the most painful navigation is along the pitch axis, I constantly need to adjust that part ("scroll the piano") to see what I need. Time axis, past and future? Sure, but to me, what matters the most is what is happening here and now.
I know this is a debatable point - time progress along the vertical axis - but I also feel it would totally screw with my perception, having to "mentally" rotate the track.
Not a big fan of having multiple "modes" either, as suggested by Raul. The way I see it, a pianoroll should be something you can simply toggle on or off. So, if there is a need for additional controls, at least don't show them until they are relevant (but this is GUI design details... )
So, when my imaginary pianoroll is enabled, it would (by default) attempt to "automatically fit", showing the most relevant keyboard range (octave) based on the notes actually present in the song. A good, general purpose fit which space for multiple tracks, side-by-side. Synthesia enjoys success with their vertical interface for training and visualizing piano notes, and I like how they managed this part.
But of course, we would need access to the full keyboard range too. So, like envelope editors and such, the piano-roll could have an optional "fullscreen" mode (also a toggle), which would take up the entire middle panel. This would initially allow you to see all notes at the same time and be freely zoomable, possibly even detachable.
This combination (ability to toggle on/off, and when active, having a compact/full mode switch) would certainly satisfy my needs. The rest lies in the details (full keyboard control, selection and such).