What would be the point of this?
Interesting, but dont see the value in it
I have never seen a UHD/BD/DVD/SACD standalone player which will work with external subtitles (including time shifting these) or even download these on-demand (features used often by me). Some also love Movie Wall features etc etc. The added value is in the many more features available on media players next to using a single device with a consistent user interface.
Also external USB-Players often have better compatibility with self burned discs, specifically if you actually get a burner and use the same device for burning (PC attached) and playback (media player attached) that big problem is gone.
Last but not least is getting rid of Cinavia, region codes and other nasty protection problems when playing copied discs.