Yes, you could customize your wall paper either before or after you hang it. I take it, you have some artistic ability that wants to be expressed and I say go for it.
Plain wall paper is available and there are stencils you could use for a repeated pattern or you could freehand a mural after you hang it. We would sure like to see pictures of the results; I think you are onto something big.
I want to redecorate my room but I don't know how I should do it!
Would it be possible to custom make wallpaper!?
I dont think you can, because Wallpaper varies greatly in its material content, which helps determine how a paper looks and how it handles. How a room is used will help you determine what wall coverings are most suitable. Get a sample or one roll and look at it in the room with your other furnishings or samples.