mpv is another fork of mplayer. Many people have asked to add support for it in SMPlayer.
In this video you can see SMPlayer playing a video using mpv:
However only very basic things work (play, pause, stop, seek).
Adding full support for mpv would be very hard. The mpv developers made a lot of changes which made mpv incompatible with SMPlayer (and other mplayer front-ends).
They renamed many options, removed the slave mode, and even removed the -identify option which SMPlayer uses to get info about the video.
Actually the slave mode is not removed, but it’s deprecated and the developers said it shouldn’t be used.
Maybe it would be possible to fully support mpv, but it would be a hard work that would take months, and only if they do not actually remove the slave mode.
What do you think? Would it be worth to try to support mpv? I mean, after all probably most of the SMPlayer users wouldn’t notice any difference compared to mplayer…