Well, how long do we think Odyssey took to produce, and how much did we know about it before it was revealed? -No doubt, if they are working on any stage of the development process for a hypothetical next expansion (including just early idle thoughts scribbled onto a napkin), we will likely not hear anything whatsoever about it for rather a while.
There are plenty of people here who express conviction that the reception to Odyssey spelled the end times for this game, but I would expect a sober company (...and the sober ones of the influential among its investors, which might admittedly be a taller order) to follow up all releases with a post mortem evaluation, determining what went right, and what went terribly wrong, and learning from this for the future; For the title in question, and for any others as well, and neither end up panic quitting, nor descending into sunk cost fallacy. I am sure the situation is salvageable, and player interest can be won back, if a popular expansion can be produced -- the company ethos seems to be to nurture ongoing products after all (...even if that all too often seems to be by way of drip-feeding it the thinnest of gruel); Even if only the most simultaneously ludicrously affluent, and passionate development houses can afford the: "playtest-to-a-fault-and-throw-away-months-of-work-without-sentimentality-if-it-doesn't-work-out" methodology, that helps those rare studios mostly avoid disappointing their player bases.
-This is not to say one can not take away the completely wrong lessons from a situation, and in perfectly good faith take catastrophically counterproductive action...
I am pretty sure I recall doctor Ross saying that the new planetary generation engine was made in significant part in order to make it relatively easy to add new types of terrain, for upcoming planet types ("draw" heightmap prefabs for formations that exhibit things like erosion, sedimentation, strata, and so on, and the rules that governs where they go), so it would seem the intention to keep rolling out new environments has at least been there at some point, and been funded.
I don't think Frontier will add any major new location types without new gameplay sets to go with them, though, which adds to the amount of work that goes into getting them out.
Personally I might reason gas giants to be a logical step before even the most barren of thicker atmosphere rocky planets... That would entail the cloud systems, weather, and aerodynamic influence on the flight model, which the latter, too, would need, whilst still defer the matter of bodies- and streams of liquids, which may not entirely be "painted" like the terrain, but should conform to it "after the fact", more than in the sense of there just being a global sea level. -That said: There would of course, e.g, be a rule that determines where a river delta prefab can go, and how much it cuts into the landscape, so they can probably get away without too much geography-level liquid simulation...
Maybe volcanic planets could get in between some points....
...and then there is the matter of whether EVA in space, and in space-bound structures, might become the focus of some expansion, cutting in between others...
