Having X or wayland USE requirements for libsdl2 is pointless.
If you can enable X or wayland USE for mpv, you don't need sdl.
Package-Manager: Portage-2.3.5, Repoman-2.3.2
)
rubberband? ( >=media-libs/rubberband-1.8.0 )
samba? ( net-fs/samba[smbclient(+)] )
- sdl? ( media-libs/libsdl2[sound,threads,video,X?,wayland?] )
+ sdl? ( media-libs/libsdl2[sound,threads,video] )
v4l? ( media-libs/libv4l )
vaapi? ( >=x11-libs/libva-1.4.0[drm?,X?,wayland?] )
vdpau? ( >=x11-libs/libvdpau-0.2 )