Fusion characters (Vegetto, Gotenks) are among many other characters that are eligable for being added to later releases, but no fusion.
Since this mod is based off of DBZ (Saiyan Saga to Majin buu Saga) no movie content or DBGT content will be added, meaning attacks, transformations and/or characters.
As RedXIII said, ESF does work with Steam. In fact it was created to work best with it. ESF no longer supports Won and will continue to create ESF to best opperate under Steam. Won versions will still be released, as long as Won isnt dead, but any problems concerning how ESF runs under Won, will not be fixed.