I wouldn't really say Ian fears for the future of the show as much as he will just really miss working with the actors and having them on TVD. Truthfully, TVD will be just fine in terms of its future.
Eh, to me Deacalion is just another villain who I'm sure we'll see again. But he isn't much more than that. His story was rather simple, which was his hunger for power. He got his sight back, that only gives him an advantage for what he wants since he's still already powerful.
A procedural? Klaus was a new character who is now getting his own spin-off. Granted he was on the show for awhile, but still, he was very much a new character introduced before.