There’s an important early step when designing a game, and that’s the initial playtests.
It’s vital to get out into the open with a very early set of rules and prototypes, and actually run through the game a couple of times. You’ll always want to do this with a trusted few play testers, those who you know won’t pull their punches.
Sometimes a game will feel great in your head, but it won’t actually play well. Rules intended to create tension and interest just lead to misery. Something that you thought was cool is just unworkable.
It’s important to actually get playthroughs as these issues become obvious quickly.
In a card game I was working on, a mechanic that was supposed to allow a losing player to catch up was just letting the leading player get further ahead. It was a bad situation, and after a couple of plays it became clear that there was no way to salvage it.
It was much better to junk the game early, and start again, then to put lots of effort into development and polish for a wider set of playtesters.
Test early and often, and iterate on what you learn. This will lead to success (or at least more fun games).