Wish you luck. Hope one day instead of Sage that is a bunch of Python glue for completely different and disjointed systems like Maxima, SymPy, Octave, GAP, PARI/GP, etc we will get a one well-integrated (and blazingly fast because written in Rust). I am a big supporter of open-source but after struggling with Sage and Co, decided that buying Wolfram Mathematica license is well-worth it (never regretted since). It would be great if we don't need to do that anymore.

As for the "most wanted" features, I would vote for PDEs, since most other systems (apart from Mathematica itself) do not support them well.