There needs to be some way, preferably in the install.xml file, that a repository can indicate which extension or plugin is most recent. 

I recently updated the script I use to make my install.xml file. Previously it only listed the latest version of each plugin, but as I expect to release some new versions that require a release newer than 7.3.0, I need it to be capable of listing multiple versions. It created an install.xml that had two KitchenTimer entries. One for version "0.7pre3" had a min SC version of 7.2; a later release, version "0.7a22" had a min SC version of 7.0 (yes, my later releases run on a broader range of SC installs). Extension Downloader suggested that I "upgrade" from 0.7a22 to 0.7pre3, which is not correct. I expect that it did so because "0.7pre3" is alphabetically greater than "0.7a22". 

There should be a way of specifying the relative age of different offerings. Most simply, there could be a serialNumber attribute in the XML that would be defined generically as a float number (much like the SOA serial numbers in BIND DNS configurations). Developers would probably simply use the zip file's last modified attribute in seconds-since-Epoch format. Extension Downloader would then only need to numerically compare serialNumber for different versions of an extension with the same name that supported the user's SC version.