Long ago, before Retail BL came out, I was making an Medieval RPG for the server because of lack of MMO-feel to the game. I built castles, regions to explore, plenty of towns, and had a system where you can buy your own property or land, buying items/weapons/etc. to help defend your territory if you're part of a Guild, and much more.
Good RPG's have content that keep the players wanting more. What do you have besides just quests and items that CANNOT be found in other servers?
Also, try crafting some dungeons and puzzles, that would keep some people as well.
EDIT: Found images of old build -

(A Farm for sale)

(A WIP that was discontinued after I left Vanilla BL; This was made by a good friend)

(Inside the town)

(Town overview)

(Castle of Hokuten Guild)

(Overview 2)

(Me visiting local church)

(Smithy)

(You know...)