What next for a fast-castler?

I've been around 950 ELO, gotten above 1000 a couple times in 1v1s, and lately I decided to just go walls and fast castle every game, practice the strategy. That's been good, I can successfully get to castle, get walled, usually deal with any early drush or man at arms ... and then lose the game lol. That's fine, I've been playing more to practice then to win, but where would the hivemind suggest I focus next? I know it depends, but is there a good practice route I could follow that builds on what I know? So far I can do fast castle and I've had some success with a pure archers build, but I've struggled to add pieces to that. Usually playing Byzantines or Britons. Or go something like Teutons/Slavs/Celts if I draw Arena and wreck shop because I've spent so much time practicing fast castle lol.