Speculation alert
I'm going to side with the latter. With the introduction of the Thememaker's Toolkit I think Frontier is signalling they're ready to move away from DLC that are predominantly theme based going forward. They're not likely to stop doing paid content updates, they just won't resemble the kind of packs we've seen previously, they'll instead allow the community to fill the gaps in this area.
It's also a bit of an omen that the Thememaker's Toolkit update will bear the version number 1.9.0, whilst it's not unheard of to go to 1.10.0 in the next iteration this tends to lead to confusion in the general population between it and 1.1.0 as people have a tendency to drop the zeros when they refer to the version numbers so many will refer to 1.10 as 1.1 adding ambiguity.
That being the case I think the next major update after 1.9.0 will go ahead and up the version number to 2.0.0 so only an epic update would do that version number justice but if they are switching up to major expansions going forward be prepared for a much longer wait between major builds.
My prediction Thememaker's Toolkit (1.9.0) around the second anniversary of the game's launch in mid-November and Planet Coaster's first major expansion (no, not water parks or zoos
) around 6 months later.