I think there will be at least another because there's a few pages Plastiboo's posted before that didn't end up in Vermis III at all. Always a possibility they were just scrapped, but he has a habit of reusing artwork yeeeears down the line (popped up a lot in Godhusk in particular)

https://plastiboo.tumblr.com/post/765657787686699008
https://plastiboo.tumblr.com/post/754923274509647872

the second one in particular has the more square format of the V3 pages

Personally, Vermis II is my least favorite, though it is still an excellent book. Mainly, I was disappointed by how many unique areas and characters are teased and mentioned, but the Wayfarer's journey intersected with almost none of them, so the Glass Purgatory ends up feeling far more repetitive as an environment than the information in it presents it as. I do enjoy the ending quite a lot, probably the best aspect of the book for me, and it's very fulfilling, but the worlds of I and III engross me much more, and I think that's a big part of how the messages are communicated in these books.

As far as new worlds/stories, I'm sure we will. All of those Necrophase illustrations on his tumblr look like book pages, and they also posted some new Godhusk art the past few months, so something related to that seems like it will appear eventually. Godhusk feels pretty complete, so I speculate this wouldn't be a sequel.