Well, I think that probably number 4# is most acceptable. Ideas 1&2 are pretty common in games, but I played some games with these systems and It soon got boring and linear (N. 2 is not that much boring, if it's well interpreted, it can be a good solution, although I don't know how it gets well with a card game). Number 3 is a pretty good system, one game is using it to unlock new equipment and it's pretty good to play.
If I'll get any more ideas, I'll "comment under this comment".
Happy developing :)