Similar app name with non-competitor, will review be successful?

Even if we knew what X is supposed to indicate, no one here can predict how App Review will react.

My opinion is that 'similar' is a risk.

See Section 4, Design,in the ASRGs:

4.1 Copycats

• Come up with your own ideas. We know you have them, so make yours come to life. Don’t simply copy the latest popular app on the App Store, or make some minor changes to another app’s name or UI and pass it off as your own. In addition to risking an intellectual property infringement claim, it makes the App Store harder to navigate and just isn’t fair to your fellow developers.

But if you haven't staged your app yet, for all you know, X is already taken...just because you don't see it in the store, doesn't mean it's up for grabs.

Most likely this will not be a problem.

The question is one of trademark law. If the apps are in different industries and cannot be confused there is no trademark infringement. I suspect App Review will never notice.

But...I also suspect that X - Have Fun originaly used the name X and discovered that it was taken.

Thank you!!

Do you know how to make sure is X taken already?

Add an app to iTunes Connect, then try the name...if it's taken, the process will complain and stop you from going further.