Thanks to @zlakergirl357, who made the project tips in official SMBX forum.
Here are tips for what to do and not to do when you're making a project. You can also post your own tips.
1. Don't have long levels unless there is between 30-50 levels in the game. If they're short make up to 50+ levels
2. Secret paths are recommended, so that your project won't be boring to play.
3. Have a variation of levels. Not every level has to be themed to the world.
4. The plot doesn't have to be long and mysterious. Even good games repeats the story from past Mario, which every Mario fan knows.
5. Get inspired by others levels. Even the greatest level designers can get inspired by others.
6. Try to come up with new features or gimmicks in your game.
7. You don't have to use new graphics. So long it fit together it's no shame.
8. Try to make bosses appear in some levels. And try to tweak them. That makes them fun.
9. Try to not have a lot of worlds in the game with a lot of levels.
10. Have fun making your project!
I hope you can understand those rules. Thanks for reading and good luck on making a project!