Actually, most of our current subtitles (especially for TV series) come from wiki-based subtitle databases like addic7ed. They're proven to work (edits are not anonymous like the wikipedia, and bad users are banned).
A versioning system is a good idea for several reasons:
-It allows to have just two subtitles for the same video file and language (one for hearing impaired, one just speech)
-It allows for the same subtitle text to exist in different timings (so videos slightly off have the same quality in subtitles as others).
-It allows for the actual creators, translators and editors to be credited and their work unequivocally related to them, instead of slapping sloppy credits in the subtitles themselves.
In these systems you *can* edit online if you wish so, but you can also download a file and later upload it again. As long as you only either edit the timings or the subtitles the system can perfectly handle the upload afterwards (the file can even be "reserved" for a couple of days to allow this).
Implementing this is less of a problem than it seems, since the whole wikisubtitles sourcebase can just be obtained and implemented. The only hooks needed in OpenSubtitles would be:
-Link for registered users to edit or create a new version of existing subs in the wiki
-TAG for versioned subtitles so they're preferred over other ones.
-OpenSubtitles always using the "tagged" subtitles from the wiki, not the database.
I know this is not simple as such, but it's much simpler than implementing a whole system from scratch.
An added benefit is that the format could be standardized as well.