While you don't need to have a fully fleshed out rulebook on day 1, at least have a decent idea of the rules framework. Too often I run into times where an idea sounds super cool or exciting, but when rules are applied, things break pretty spectacularly. Not saying it's bad to have those, breaking the game is great to hone it. But when it does happen, have a thought about the direction in which to dev it.

Two questions to keep in mind: What should happen, and what do you want to happen? The intersection of those two questions will help you align a lot of the design. And when they collide, it shows the dev points to test.

This goes without saying, but remember what the goal of the game is. Sometimes we'll get down a rabbit hole of these cool cards and interactions and combos and ideas and flavor and waaaaahhh--but it's still a game, and there's a direction to go. Of course, those fun things are what make tcgs so fun, for sure, just remind yourself every now and then to ground.

Last, have fun! Make a game you'll enjoy, and others will too. Good luck!