I for one would make great use of this, if it can be implemented.
Other areas that I believe that should be improved in addition to having Bezier Tempo curves, should be a List Editor for the Tempo Map itself, (similar to the editing of MIDI data), as well as quick editing in the Inspector (similar to Markers) as well as of course having Automation data being editable via a list so that quantization is a relatively simple process.
Steinberg have given users so much in recent years, it’s difficult to imagine something like this is not on their minds already.
As for Bezier in the Key Editor, how about just allowing curves to be edited there, that originate from the project page (or vice-versa) and not worry about MIDI 1.0 information anymore, other than translation between data in the DAW and MIDI exports.
Best, all!