Building castles "for funsies" is certainly an option. Another option is to take a break, or try recruiting some friends and delve into the world of dedicated servers. Or you can explore the mod community. Or you can explore the mod community with friends.

Or you can make a new character and start a fresh seed.

Or you could use your experience to write some guides or start you own streaming channel.

The options are vast. (and they could even include never playing the game again)

It really comes down to personal preference.