Diminishing returns by which I guess you meant that most of the good ideas have been done and so a further expansion would contain less good ideas and variations on ideas which had already been done.
No, diminishing returns means, when you have 25 cards, adding 25 with Intrigue doubles your number of cards and drastically increases your variety; but once you have 200 cards, you have a crazy amount of variety, and adding 25 more adds a lot less. You still get to explore the expansion itself, see what the new cards do and have those experiences, but in terms of giving the game variety, there are diminishing returns.
I find myself wondering does this mean you actually have enough good but really complicated ideas to make up another expansion (but feel that there would be no market for such an expansion, or that such an expansion would alienate casual gamers from the game)? Or are there simply not enough good ideas for cards left to do another a-grade expansion whatever the level of complexity?
There is also the insularity issue. A typical direction to go for an expansion is more like Alchemy than Intrigue. Let's say there's a Puerto Rico expansion; it adds plantations and corn and indigo and stuff. This game there's only one card from it and no-one bothers with it and people say what is this nonsense, worst expansion ever. But a spin-off can just make the plantations etc. a main part of what's going on.
If complexity and insularity aren't issues then you can make expansions forever. It isn't just the casual gaming public that doesn't want hyper-complex insular expansions though. Alchemy isn't too complex (it's too slow but that's obv. not the same thing) but it's too insular for too many people, despite struggling not to be. We don't have the data on the level of complexity Guilds (a non-insular expansion) has yet; obv. I am hoping it's not too complex, but it certainly points the direction that things to come would go in.
It's not that there's a big list of ideas and I can just say, too complex, too complex, too complex. There's a big list of ideas; I gave a sample in response to an earlier question (
http://forum.dominionstrategy.com/index.php?topic=5799.msg149323#msg149323). A typical idea just wasn't worth doing; there are a variety of possible reasons. There are ideas that probably would be fine but wouldn't be the exciting thing in their set; I could scrounge up a dull expansion. There is also a list of general mechanics; there most things are too insular, though they may have other issues.
If I were making a new expansion, it would mostly be new ideas, not stuff from the list. Maybe I would take a few things from the files, but just a few. And I did this, I made a new expansion, just like you want me to; it's Guilds. There were "only" going to be seven expansions, but I "needed" a small expansion to go between Hinterlands and Dark Ages (which then didn't go there due to Base Cards). I picked the most promising mechanic on my list and made some cards and well I expect the expansion to go over well with Dominion fans. And it's mostly new cards, with a few Dark Ages or earlier cards that I either moved there or finally fixed up there. And it's the most complex expansion. It's not "old ideas on the list that were too complex," it's, "try to do something good that isn't too redundant with what's already out there, hey look at that, it's complex."
Another item you left off your list is, that making a Dominion expansion means not doing whatever else. I had fun making the expansions but would like to make other games too.
