I think party of ninjas is only reason to get this game, but maybe its not enough. Yangs daughter seems worth of seeing too and teen Palom and Porom but then again this game doesn´t seem interesting at all. Only good direct FF sequel is Revenant Wings.

Kain´s story is only worth to get sequel but I guess they could have worked him somehow into Dissidia. It may not be canon but still it would have shown how much he wants to restore his and dragoons honor.