I always wondered what a sequel to The Doomsday Machine would be like. I know that when they chose to do a sequel (The Wrath of Khan) for Space Seed, part of the decision may have been based on the following exchange between Spock and Kirk at the end of Space Seed:
SPOCK: It would be interesting, Captain, to return to that world in a hundred years and to learn what crop has sprung from the seed you planted today.
KIRK: Yes, Mister Spock, it would indeed.
I noticed that at the end of The Doomsday Machine, they have similar dialogue that could very easily have suggested a sequel:
SPOCK: Appropriate, Captain. However, I can't help wondering if there are any more of those weapons wandering around the universe.
KIRK: Well, I certainly hope not. I found one quite sufficient.
So, what do you think a sequel to The Doomsday Machine would have been like, either as another episode in Season 4 or 5 or more than likely, as a movie (instead of TWOK) - STII: The Doomsday Ultimatum?
I could imagine a bunch of different scenarios:

Maybe if we ever get another Trek series on TV, we'll finally get the sequel.
SPOCK: It would be interesting, Captain, to return to that world in a hundred years and to learn what crop has sprung from the seed you planted today.
KIRK: Yes, Mister Spock, it would indeed.
I noticed that at the end of The Doomsday Machine, they have similar dialogue that could very easily have suggested a sequel:
SPOCK: Appropriate, Captain. However, I can't help wondering if there are any more of those weapons wandering around the universe.
KIRK: Well, I certainly hope not. I found one quite sufficient.
So, what do you think a sequel to The Doomsday Machine would have been like, either as another episode in Season 4 or 5 or more than likely, as a movie (instead of TWOK) - STII: The Doomsday Ultimatum?
I could imagine a bunch of different scenarios:
- A dozen Doomsday Machines attacking our galaxy, sent by the race that built the originals. The newer ones are much more advanced.
- Will Decker somehow returning and playing a vital role in defeating them (That's for my father!!)
- Time Travel (ok I had to say it) used to return to the original battle, possibly saving Matt Decker and obtaining more information needed to defeat the new machines. Matt & Will sacrificing themselves (again) to defeat the new machines.

Maybe if we ever get another Trek series on TV, we'll finally get the sequel.