GMing Makes You a Better Person: Game Designer
The first time I introduced a custom mechanic into one of my games was way back when I started, in the system Dungeon World. My friends and I liked the allure of Dungeon World because it was very simple to get into and play.
I had each of my friends make characters and each time they leveled up, I'd have them create a new "move" for each of the other players based on their character. I would then add an ability myself and they would get to choose one of the three options. For example, one was a fire themed wizard so the other players and myself all made him special fire attacks, and he got to add one to his repertoire.
This worked pretty nicely and got each of us talking about the powers we liked or would like in the future. It was really nice getting custom input from the players and also throwing my own ideas in to the mix. It was even more satisfying creating a system that the players were able to enjoy.
Systems Thinking
As you continue to tweak your game and make things for your players you'll become better at thinking in systems. You'll anticipate ways things can go wrong and edge cases you'll hit. You'll also learn balance and arguably most importantly, how to say NO to things.
I once had a player who played a necromancer and had graciously found his own home-brew set necromancy themed of spells. I now imagine myself as Matthew McConaughey screaming at his past self through the bookshelf in Interstellar to not include those spells.
They broke my game most sessions until I had to have the discussion with him to remove them and try to reskin spells already balanced for the game. It was a great learning experience and both the player and I look back and laugh at it now. Don't be afraid to say no.
Your Personal Touch
Most of the best parts of your game will be the custom things you add in. Because you're the one who knows what your players want, not the people writing adventures. I'm not saying everything has to be home-brew, but in my experience, my players have loved my custom additions most of all.
You can tailor things to their expectations and their likes or dislikes as well. This will help you discover what they enjoy or what they find fun or frustrating. You'll get comfortable with sharing imperfect ideas, receiving feedback and develop a sort of empathy with your players.
A Labor of Love
Game design isn't something reserved for professionals. If you've ever created a magical item, changed a rule, or made a new mechanic than you're already doing it. And every time you do it, you get better.
So give each of your ideas a shot, whether it be a home-brew spell (maybe be extra careful with necromancy) or a weird rule that makes everyone laugh. TTRPGs are best when they reflect the people at the table and that's something no published adventure can do for you.