I'm kind of lost right now. I don't know what else to learn next, regarding guitar theory. I know my major and minor chords, notes, notes on the fretboard, Major scales. How to harmonize scales. And I think that's it. What should I learn next? :/
You only know the major and minor chords? I'd start exploring other chords now then.
Key theory and scales. That will help you learn which chords belong in which key, and scales will help you improvise and write melodies within those keys.
And no, Guitar Hero will not help. Even on expert. Really.
You should learn different types of chords, like 7th, 9th, etc. etc. And you should probably learn minor scales and modes of both major and minor scales.