I came across this thing that's look crazy in my eyes.
if I understood well, this is a mod to replace the textures of sonic adventure by more beautiful from the DX version released on PC:https://www.moddb.com/mods/sadx-dreamcast-conversion
With a little tutorial :https://www.moddb.com/mods/sadx-dreamca
It's quite recent since the last update dates back to August 2018! For a first version that was released in 2016.
Unfortunately I couldn't get it to work...
If it was applicable to the version "sonic adventure international" it would be great !
Please help me ladies and gentlemen.