Dominion allows players to choose which card types to include in each session, giving the game a great amount of variability. There are also suggested setups. How did you determine what options to give players?
I tried to keep the set-up simple. The simplest thing is to have one kind of thing that varies, so I try that first. For some games you might be stuck having multiple kinds of things to vary, but this wasn't necessary in Dominion. For Kingdom Builder I vary the scoring separate from the boards.
I try to have games vary as much as possible but not too much. I don't want it to change so much that it's not that game you like, but otherwise I want to really shake things up.
My expectation is that, when there's a set of things to pick from randomly, many people will pick randomly! So that does affect what things are in the set; for example, if something is no good for new players, it might be stuck waiting for an expansion (or never happening). If a combo is too dominating, you might have that combo, so I have to fix it.
The suggested setups, after the first game one, exist because the thought was that some people might like them, as a way to not just jump into pure random. Originally I did not put much work into them; after all, the game is supposed to work with random cards. So I picked cards that went together in whatever ways and they were played once each and that's that. For much later sets, it was clear some people played all of them, and I started playtesting them and tweaking them a little. They are still not all that polished; again the game is supposed to work with random cards, and if it does then these lists will work too.
How much guidance should designers give players in variable setup? Do they need to be wary of an abundance of variety creating an unpredictable experience?
It's a concern in two ways.
First, you want the first game to be as good as possible. If you leave it up to chance, it will sometimes be the very worst case for a first game. So, just specify what portion of the variable setup to use for your first game. Dominion says, play with these 10 cards.
Second, there are all the other games, after the first game. If there's something you need in the game, you need to make sure it's there. For example if it was important to always have a +2 Actions card in Dominion, and I didn't want it to always be the same one, then there would need to be, say, a separate pile of those, so you took nine random regular cards plus one random +2 Actions card. It's nice to avoid having to complicate the set-up like that, but you may have to.
Players can play with whatever cards. If they pick randomly they'll see certain things as often as I like them; for example, a typical game will have a +2 Actions card, but some games there won't be one, for a different experience. But some players may wish to always have +2 Actions, or always have a defense if they have an attack, or whatever, and that's fine. They'll know how much they like it and can stop doing things they don't like. They can always go back to random.
The cards in the game almost all directly help you build up. So it's hard to have a random set of 10 that doesn't give you ways to move forward (without just buying treasures). A big trick here is that smaller effects will have basic resources attached, like the +$2 on Militia.