Starting anything from scratch is a huge risk these days. At best you’ll have something like the python 2 -> 3 rewrite overhaul (leaving scraps of legacy code all over the place), at worst you’ll have something like gnome/kde (where the community schisms rather than adopting a new standard). I would say that most of the time, there are only two ways to get a new standard to reach mass adoption.
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Retrofit everything. Extend old APIs where possible. Build your new layer on top of https, or javascript, or ascii, or something else that already has widespread adoption. Make a clear upgrade path for old users, but maintain compatibility for as long as possible.
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Buy 99% of the market and declare yourself king (cough cough chromium).
syncthing is the easy option if you have some files you always want to have on both. if you just want to access your desktop files from your phone, I recommend Cx File Explorer for Android, it’s a file browser that supports various network file share protocols including Samba and SFTP.