QuasiUnaFantasia Great idea.
Kawai - improve build quality. Improve touch response (it's always kind of exaggerated/jumpy). Improve samples, the bass notes are too bass-guitar like. Improve sound system, it tends to be boomy and V-shaped.
Yamaha - how about having two variants of each keyboard action, a heavy one (classic) and a light one. Maybe even devise a mechanical solution, so that the action weight can be altered easily.
Roland - abandon modeling ASAP! Go back to sampling. In addition to overly-processed pianos (the Roland trademark pop piano sound), provide fine and balanced samples that are suitable for classical music.
Casio - fire the entire piano sound/engine/sampling department ASAP! Apparently they love plinky piano sounds. We don't! A rename for their digital pianos wouldn't hurt either 😉
KORG - maybe focus on what you can do: synths/workstations, not pianos.
Studiologic - improve quality, test firmware more, hire classical pianists to work on the touch response and how the entire dynamic range should sound.
Nord - halve the price 😀 Also, hire classical pianists. The last time I tested a Nord Piano, it was an underwhelming experience.
Everybody: start using huge amounts of sampling storage for non-looped samples with enough velocity layers and distance mics for added ambience, rather than cheap/fake DSP reverb. Also, come up with digital piano actions that emulate escapement (not escapement bump!), so that there's no constant pushback against your fingers.