This is kind of hilarious. Not long after the details of the new Ghostbusters DS game were revealed, describing a game that sounds extremely similar to the old Ghostbusters game, a French homebrewer calling himself sephiroth_FF7 released an alpha of a DS port based on the Sega Master System game. In essence, he's beaten Sierra to the punch. This isn't some quick port, either. The game is based on elements of different versions of the game according to sephiroth_FF7's own preference, while the graphics hew closely to the Master System version.
Of course, this raises the question of why someone would go to that much trouble to remake this game. But more importantly, why is Sierra going to even more trouble to do so commercially?

