I've been obsessing over my latest game, Crystal Clash, tweaking it and playtesting it almost daily. It's been getting better with each iteration, but it was still overly restrictive and didn't allow for the simple, emergent style of gameplay I was striving for.
Then I thought to myself, "What if I remove the mana system (crystals as it were) that governed what players could play?" It was a radical change that would change pretty much everything, but with it gone, it allowed for increased player agency while also simplifying the entire design. This change, along with another minor change, vasty sped up the game, made turns far more exciting, and decreased complexity and downtime.
My big takeaway is that you need to eliminate mechanics that aren't in service of how the game wants to play. I liked the idea of saving up crystals to play cooler monsters, but that one cool idea was dragging the good ideas down. See what happens when you hack mechanics off that aren't essential. You may find a better game.
Now my biggest problem is that my game needs a new name, but I think that's a good problem to have.
I hadn't thought of a mechanic like that, but what makes the game shine at the moment is how synergistic all of the minions are and finding fun ways to play them together. Limiting which minions a player can play makes it less open-ended and creative. And since they all cost nothing, I can make them all super splashy and fun to play, because they're all powerful. Games only take around 7 turns, which are simultaneous, so it's fast and fun. Currency only slows the game down.
But I have to remember playing cards as mana for a future design because I really like that mechanic. It's a fun choice to make.
EDIT: Actually, I could design a hero in my game around that mechanic. Hmmm, good food for thought.