I would first vote for the outside workout features. I think this should be number 1 because most people probably train inside so they can improve their outside riding/racing.
My second area of interest would be incorporating data from fitness deives. Even an old crumugeon like myself has an Apple watch and there is a ton of probably useful data recordedthat could guide progression and probably even more importantly assess your need for rest.
I am not sure I would find the FTP estimator all that helpful. I kinda like the ramp test. What would be cool would be displaying the your FTP as you do the ramp test. This number would be the ramp test value if you were to stop at that moment. Even better would be having the ramp test FTP value displayed against the estimated value. I am pretty sure I can hang on for another minute of the ramp test if it means beating Amber’s estimate.