Hi,
My idea would be the following. It's similar to the wiki approach, but a lot simpler. People could upload updated versions of subtitles.
The example step by step:
1) download a subtitle
2) see there are some errors, for example lower/uppercase
3) fix the errors
4) when reuploading the subtitle, instead of making a brand new one, you could select which one it's based on. You'd also have to say what was fixed.
5) from then on the site would offer the newest one by default, if someone clicks on the main download link, but the older versions would be available as well