Demon's Souls was one of those games with a really unique co-op premise.  Instead of traditional forms of co-op, players could be randomly assisted by other players that have died in their own game.  Not only did the Atlus game go on to receive great reviews, it went on to sell well enough to earn a Greatest Hits release on the PlayStation 3 - something the publisher rarely sees for its niche titles.  So with good sales, should come a sequel, no?