Is the idea still to keep continuing to create new expansions at Rio/Jay's request or are you eventually done introducing new ideas/mechanics?
You are assuming that RGG/Jay is requesting expansions.
Well yes, this is based on an answer by DXV I once read (can't find it unfortunately) that he had a number of expansions set from the get-go and would only do others if Rio/Jay convinced him to do so.
I really wish I could find that answer.
I have said something like, "Publishers will want more at some point and I like to be friendly." I have had that experience with both Kingdom Builder (the 3rd and 4th expansions) and Dominion (Alchemy jumping ahead and being small) (and promos for both, those never start with me), but Adventures and Empires came about in similar ways, due to me feeling like I wasn't getting anything done on other projects. For Adventures that was it; I'd puttered around some on spin-offs, and then, okay, a Dominion expansion. For Empires, there was a point where I considered what the good future space for Dominion was, and came up with some stuff, and then it was like, okay, nothing else is going on, why not try some of that. But in both cases it was me, wanting to be getting work done on something.
The idea as always is to work on new games, to not fall into this trap again. Given my past experiences it seems likely I will though. I will try to put it off, but at some point, it will be what I can get done and I will do it.
Demand goes down with each expansion. People think, three expansions is enough, or five, or however many. It may still stay profitable for RGG to make more for years and years. For me, that part isn't even an issue; I think at some point it could be that another expansion wasn't going to ever come out, and I might make one anyway. They're fun, what can I say.
Having enough reasonably simple stuff to do is an issue. Cards are more hemmed in than ever by what's already been done. It might just be a few more expansions before it seems too hard to make stuff that isn't too obviously better/worse than something else at the same cost, too hard to make stuff that doesn't require extra components, too hard to make new things that fit on the cards. But I won't really know this until I'm there, struggling with that expansion.