Orson Welles had it that "The enemy of art is the absence of limitations." And to me Dominion absolutely exemplifies this. Take Treasury, which as you mentioned earlier, is sort of like a permanent Duration card. Instead of writing up new rules for a new card type, you shoehorned it into the rules framework you had. And from this you get depth: it develops interesting interactions with discard attacks, it's a guaranteed target for Thrones or Graverobbers or whatever, all kinds of stuff. I think it's reasonable to say that a lot of the nuance in Dominion comes from stuff like this, and of course you've been making Dominion cards long enough to appreciate it (though maybe long enough to get sick of it).
So this isn't a criticism of course; obviously you know more about game design than I do. And obviously it's not like deckbuilders are the only game format that provides some basic structure to work from. But I'm surprised to hear that after your Dominion experience you find "limitations" to be a dirty word. Do you feel like pushing against boundaries is a major part of your design process? Or is it the sort of thing where like, your game mechanics are ultimately going to limit you no matter what you do, so you might as well carve out as big of a design space as you can?
Mark Rosewater says, man how does he put it. Restrictions breed creativity. If you need inspiration, man, order some up. One way to be inspired is to box in your possibilities and that's fine. Sometimes you get inspiration some other way; that's fine too. It can be hard staring at a blank page, and in the end you will have something very specific on it; we can view the task as entirely one of cutting down the possibilities. And if you can get part of the way there that's better than uh not getting anywhere. Anyway you know, that's all well and good although it doesn't mean you constantly need restrictions. Sometimes you've just got good ideas, you leap right to some good stuff on that page. Restrictions in this sense are a tool but not the only one. I mean you're always restricting things but that's not always the clearest way to look at it.
My games tend to work with as little as possible; they are heavily restricted in that sense. In Dominion your VP go in your deck, your money is in your deck, your actions are in your deck. The reason I went that route was simply to try the most extreme version of the idea. In Kingdom Builder you place 3 pieces on your terrain, adjacent to you if possible, gain abilities when you play by them, can't use them the turn you get them, and draw a new card at end of turn. Most of my games are low on rules and can be taught very quickly. It's a trick because there are rules on cards, but you know, the framework is minimal. If there was something I didn't need, it's not there. And I see how much I can do with what little I've got.
In some of my games there will be this real question of, can you make enough cards for this. The number of cards you can make depends on the complexity of the cards and the amount of rules you have. When you don't have many rules, the pressure is on the card text, which tends to want to be simple too. I have tackled this so many times that I know a lot of basic things you can do with almost nothing. Let's say you have points of some kind, monkey points. Well you can gain monkey points. You can make the other players lose them. You can do both at once, always satisfying. If there are lots of ways to gain them, I can make a way to increase how many you gain, and if there are lots of ways to lose them, I can make a way to avoid losing as many. There aren't a lot of things you can do with just monkey points, but you know, a game without many rules can have more card variety than you might think.
But none of this has anything to do with "should I make a purely digital card game." That restriction isn't interesting or new or anything. There is better territory to stake out. Like, when you are trying to fill that blank page, deciding not to use the letter e is not a great start. It's a restriction but it's not doing good things for you. Someone already wrote something with no e's, and man no-one needs to read it.