A Good Dungeon Master must be a good Game Designer.
A D&D important point is that it gives a wide flexibility and personalization of the game.
You, everyone, want to introduce new rules, change something and use House Rules, and while you do so you automatically become Game Designers.
You must act like that.
To do not carefully means to break the thin balance of the game mechanics.
Let's see how to avoid it
Learn well the mechanics in the game's official books
How can you change or add or delete something if you don't know the official rules and the hidden motivations behind them?
Therefore learn and use for some time the official rules. You can understand where you have to work.
The game system is already a good game system
It can be D&D or another game, but always keep in mind that its game system has been developed by many professional and experienced designers, who have worked for many months and have playtested a lot.
You don't need to change dramatically its mechanics.
Make a change at time
Make few focused changes, one at time.
Make too many at the same time can seriously put you into troubles because you would have too many things to follow.
Test what you have just created
New classes, races or other (spells, magic items) should appear for the first times only as NPCs, allies or enemies of the players.
You will have then the time to analyze their impacts on the game.
Every innovations of your can always be revoked
Even if you follow what written before you can always discover in a long period that some innovations are not suited for the game.
Make clear to everyone that you can always change or delete what you have created.
Innovations must be worthy
What benefits does bring your change?
Ask yourself it anytime you're creating something.
If you cannot answer then drop it.
You innovations always should:
- solve frequent situations happening in your game which aren't covered by the rulebooks
- be about some very important aspects of your game not covered by the rulebooks
- improve your players' satisfaction