The Sierra brand and rights are now part of Blizzard/Activision, i doubt there are any right/licensing issues. I think they would allow it too, but i believe there are some hardcore tecnicall issues with that Expansion. The way Synergistic implemented Hellfire was not ideal, since Hellfire has all Original Diablo content without actually being part of Diablo installation, it's a separate game on it's own. Maybe Hellfire would cannibalize Diablo sales unless they find a way to check if Diablo is installed, which would be kind of a DRM. There are also technical issues because the game was based on an older Diablo version, 1.03 or 1.04 something like that... maybe they would need to update Hellfire to be on par with Diablo 1.09b, and i am not sure Blizzard wants to put any cash for a title that is not quite loved by them. Also, Hellfire MP characters are located in Windows folder and are also hidden, which creates a headache trying to back up the files. I love Hellfire, i really hope they find a way to make it work since Diablo in my opinion is not complete without it, i would pay another 5-7 USD for the expansion, i totally would.