As with bug 43149: Allow MPEG and WMV uploads to Commons: transparently convert to the preferred free media format and return that as the final file (and filename extension) on Commons. (For bonus points: support conversion out of RM format as well)
This will add support for the vast majority of all video files in existence, and will allow people who don't understand what "transcode" means from sharing free media with the world.
The Internet Archive provides this service if we don't want to run converters ourselves; we simply have to handle the pipelining to and from IA, who would be happy to see that traffic (and host a copy of the files to boot).
T45149: Commons uploads: Support automatic conversion of mp3 formats to free format