Just because an idea is good, doesn’t mean it’s good for this game.
All of the designers I know have more ideas than they can get finished, and this is a good thing. It shows a fertile and creative mind. I find ideas are a bit like the hydra of old: get one into a project and published, and two more spring up in its place, begging to be used too. This abundance of ideas makes it very tempting to crowbar as many into each game as possible, and this is a trap.
Less is more is a somewhat trite cliché, but under the rather shop-worn exterior is an important concept. Sometimes taking things away allows the real strength of the core concept to shine through. Sometimes all the extra “chrome” isn’t adding at all: it’s just obscuring the really good parts. Always, the trick is to know when and what to prune.
This is a bit like murdering your darlings, and can often include that jolly concept, but it is more than that too. What I’m talking about is focussing your design on what is really important for this game. In that process of honing you should definitely murder excess darlings, but you should also look critically at everything else too. As I said at the start: just because an idea is good, doesn’t mean it’s good for this game. And that’s the critical point: for this game. Many, if not all, game concepts and rules are great in the right place and a problem in the wrong one. It’s seldom the rule itself that is at fault, merely the juxtaposition of it with the rest of the design.