Gamera
The series left behind any pretense of being a science fiction series and it's totally fantasy at this point. Who or what will pop up next?
This can be the fate of too many shows just "left to run" without any thought of an over-arcing story arc, or some reasonable point to move to, and then from.
It's endemic in the SF genre, and one that particularly plagues episodic SF. The writers get caught up in the heroic ensemble cast, and fail to give equal consideration to their foes.
If you stand up an implacable foe, even worse, and invincible enemy--what happens when your heroes win? This is particularly complicated if you have a resolution in mind, but you get canceled a season before you can carry that off (see Farscape, among others). It's worse if you have no resolution in mind, and get continued for another season.
Also, the production companies need to understand, and believe in the success of serials with multiple, interleaved, story arcs, to where an episode can be "padded out" with forshadowing and left "hanging" at the end. J. Michael Strazinski really proved that with Babylon 5.
He built a five-year story arc, and put not one, but two "end points" in the middle of that, to allow for early cancellation. Until B5, most "tv people" assumed you "had to" resolve the entire episode in the episode, or no one would come back next week.
The notion was slow to "take"--witness Andromeda, one of Gene Roddenbery's last, and helmed by his widow, Majel. About halfway through Season One, they started writing in deeper arcs, and incorporating parts of earlier episodes as foreshadowing (if after the fact). S2 uses those more effectively. Which is what carried Andromeda through 5 seasons.
It's the "grit" that kept The Expanse going, despite SyFy canceling it (or, perhaps, because of). Interweaved story arcs is the definition of Farscape, too (also cancled by SyFy for being "too successful").
What do you do with an implacable foe after you defeat it? Well, SG-1 found a way, a brand new enemy. (That is, until SyFy, itself, became the enemy of the series).