User blog comment:The Imperial Ghost/SpongeToons episode preview! Guess which episode!/@comment-24438956-20140831185224

Time Rift likely, because:

We haven't started writing the ST movie. We're co-writing it and I said that we'd start when we're both ready.

ST: 50 Episodes is a celebratory episode and has nothing to do with that script.

You stated Ghastly Operation comes this Wednesday and it'd be too obvious to actually have that be the episode.

So Time Rift is my guess.