I'll give it a 0% chance of being anywhere near 200 chapters. This is not a weekly manga, it's a monthly one, so it should be compared to monthly chapter formats. Another manga with a similar level of popularity is FMA. That ended up being 108 chapters, and 27 volumes. Shingeki is almost half way there, and it might or might not be as long as FMA.
FMA is a very bad comparison, since it got an anime really early on. By the time it had 11 volumes or so I'm pretty sure it was just as popular as Shingeki is now. The circumstances are just different.
As for Araki doing sequels, he has nothing against that I think. IIRC, he did say he was willing to do more Highschool of the Dead, but someone has to pay for it, and there needs to be more manga. Lol.
Yeah, with FMA, lots of original material was unavoidable because they had so many episodes planned and not nearly enough manga to adapt. They're not going to reach the end of the available manga with Shingeki in just 26 episodes, so any original material would be by choice.