"So the last time players saw her was at Shimousa and it was assumed that she died."
She did die. However, there's an epilogue in Shimousa that reveals that she's become a Heroic Spirit, and the PC was apprised of this in a letter that she left.
Well for it to be made into an anime it would require it to be consistently popular, so I'm guessing about half a year more or so before production comes underway