Does it make the game more fun?

 As a DM, especially one who likes to toy with the game and make it my own, I constantly ask myself this question. If I'm going to add a new class or race, switch from race-as-class to race-and-class (or back again), if I'm going to reskin everything to make it feel like Asian fantasy or Star Wars or whatever, even if I'm just adding in some new monsters, the question that's always on my mind is:

Does this make the game more fun? 

And the question has more than one answer. Who's fun will the change enhance? Whose will it detract from?

In my current state of Treasures, Serpents and Ruins, I added Dragonborn and Changelings (Tiefling/Aasimar) because I started my West Marches game in 5E and those races were popular with my players. I'm not overly fond of either one, but removing them would make the game less fun for my players. So I made simple 1E/BX/BECMI style versions of those races.

Yesterday, a new player came to my game. She thought she'd play a Druid. But the stats she rolled didn't have a high enough Con score (a requirement for Druids in my game). In order to let her have her character which she would have fun playing, I let her change the score to the minimum needed. Problem solved, she had a great time.

The switch from 5E back to Classic (with heavy house ruling) was necessary for ME to have more fun with the game. Yeah, I lost a few players. But the ones that stuck around, and the new players that joined, are having a blast. And I am, too. I'm a lot more confident running the game, and prep for the game is much easier as well.

If a change to the rules, the systems, or the procedures of gaming make the game more fun for one or more participants, and don't significantly reduce the fun for other participants, then that's a good change to make. Even if it doesn't enhance the fun, if it makes things easier for some participants without reducing anyone else's fun, it's probably a good change.

Yes, "fun" is subjective, impossible to quantify and define in a satisfying manner. Yes, what seems fun now may seem less fun in the future. If the "fun" is decreasing over time, that 's just a sign to either go back to the way it was before, or else try something new again.

If you're thinking of making changes to your game, of course you should ask yourself "Is this necessary?"  If the answer is yes, keep going. But don't give up on the idea just because the answer is no. Also ask yourself next if it makes the game more fun. Only if the answer to both is no should you abandon the idea.