You used common practice harmony, and than the most easy version of it. Mainly tonic, subdominant and dominant chords. You transpose to the parallel key, again the most obvious choice.
My advise is to learn from musical history.
Start analyzing more music from say the romantic era. See how composers move to farther tonal regions. (Brahms or Tschaikofsky, Mahler or whatever you fancy)
Then study harmonies that add or alter then tonal framwork. See for example how Debussy uses added notes. Or listen to some Russians. Mousorgsky, RimskyKorsakov, or Scriabin. they add all kinds of octatonic scale, or other composer ones, like Shostakovich, Prokofiev.
You could continue to discover quartal harmony (that is not based on triads as in the common practice era, but on 4ths)
Look for bitonality (Honneger amongst others).
See what nice sonorities you can create with serial music. Start for example with early Schoenberg, or take the violin concerto by Alban Berg, which I really love. It is a great and accessable introduction in serial music.
I hope this helps