How would you make a huge world like Pokemon?

Game Freak doesn't care if you use it's sprites. Just look at I Wanna Be The Guy and the amount of sprites from Nintendo games! And they said, we don't care.

Or just modify their sprites so I do not get into trouble with copyright?

Using Pokemon buildings is a bad choice from the start. But as someone said, you better learn. Unless you want to make a tribute to the old video games like IWBTG.

Using Pokemon buildings is a bad choice from the start. But as someone said, you better learn. Unless you want to make a tribute to the old video games like IWBTG.

You may be right. What I thought of doing was using the Pokemon buildings as a template, making my own creations out of them and heavily modify them so they do not look like Pokemon buildings. It teaches me to learn the aspects of pixel art, and that's only one step of numerous I have to do. I still will have to make character sprites and others. But I think of it as a good start.