To the degree that "duration" is a new type, sure, any new expansion could have something like that. I would also consider revisiting duration cards themselves; the people excited by a 9th Dominion expansion would probably be happy to hear it had new duration cards.
So this is in relation to your talk about the possibility of doing more duration cards (until Jay suggested something new), as well as your detailed explanation of the new VP card possibilities.
Would you ever consider doing more VP token cards as an alternative/supplement to more VP cards? They seem to fill the role of providing new paths to victory without really clashing with other stuff (for example: if Duke and Feodum were in the same set it would seem redundant, but you have 3 VP token cards in a single set without them really encroaching on each other). It seems like all of the "classic" things you can do with VP tokens were used, but especially with new mechanics like overpay and on-trash that there are some possibilities for new things.
It does have the duration problem of "isn't a new thing better", but it seems to provide new routes for "how do we create different victory paths since all the classic VP cards are in sets".