It would be great to have the video stream selectable - same as is audio now.  It appears to be getting more common to embed alternative streams (such as 1080p) alongside a UHD stream particularly within MKV's containers.  This feature would allow us to select a 'best stream' for a given device, an example recently being my Samsung TV not handling 60fps which was the default stream in the MKV.  A 1080p was available also but could not be selected, so my server had to transcode a large UHD high-f