If I may chime in (late), I think a subsctiption system is a bad idea because experienced but occasional players will disappear.
Forcing new freetoplayers into their own, Base-infested ghetto is not a great way to keep those players if they don't shell money in the first week of play or so (even adding the occasional "on trial" card to the free set - say one or two different cards per week - Base Dominion gets stale pretty fast).
I, like many others before, suggest selling cosmetical upgrades for money - and not just the base cards. You could sell alternative ("shiny") versions of existing popular cards (with less popular artwork - say Harem), backgrounds, avatars, personalized cardbacks...
In general, people are more likely to buy such nonessential stuff if said non-essential stuff is visible to other players.
You could make alt-art skins for coppers and estates, and people could buy them to personalize their starting deck. So, no in-game difference as cards only care about names, but your opponent will be green of envy when you play a bunny copper, a limited edition Adventures pre-order copper, and a "Tournament: first place" copper to buy a Silver, while all they have in hand is five lousy, cheapass coppers.
(maybe add the rule that they become vanilla coppers and estates as soon as they leave your ownership, to avoid masq/ambassador weirdness).