The Problem: A player wants a new power for their character, which may be a spell, a class, or some new species that can fly or shapeshift. The game master thinks this option is overpowered.
The Duct Tape Solution: You could just say no, but that can cause bickering and/or resentment. It’s better to point out some of the reservations you have. Be specific—explain why you think there’s a potential problem. Then ask the player how they want to ensure their choice of power will be fair to the other players.
You, the game master don’t want one player character to overshadow the others. Neither do you want the new power to spoil challenges that you and the other players think are fun. If the player is reasonable, you can work something out.
For example, suppose one player has a new book featuring avian humanoids who can fly, and wants to play one of them. Whether this is a balance problem depends on whether all the other characters can fly or teleport and what it costs them. You could point out that it looks like this species has the ability to fly at will, and that it would cost anyone else in the party one of their good spells to do the same. You could also add a flying player character makes obstacles such as chasms, cliffs, and pit traps irrelevant. Ask the player which other characters are most likely to be overshadowed by their character’s flying ability, and how they’ll make sure those characters get to do the fun things they were designed to do.
This is a lot easier for you when the overpowered idea is coming out of the player’s imagination or some random thing they found on the internet. You have to be assertive if the player has a rule book to wield against you. Just because a power is written in a book, doesn’t invalidate your concerns.
In an ideal game, the rules would help make sure all the players have the same opportunity to do fun things. When the game designers fall short, or when their idea of balance is simply divergent from yours, it’s up to you to ensure fairness at the table. Sometimes you have to act as a referee, blowing the whistle to stop a problem. It’s usually better to act as an adviser, helping players to see how their choices affect other players at the table.
