Riku probably won't be faught again, they were streching it as is in COM, lucky about the Replica thing... Anyway the only boss that probably won't come back will be Ursala and Clayton, because they both are gone for good. The others are tough or cool enough to come back, or didn't die good enough. Sephiroth is a most, he was WAY too easy for my tastes. A good placed Ars Arcanum whopped him good.