Hard to say as I don't know exactly how you envision the world and story but I'd start by saying more areas. Maybe have them branching off so you can tackle them in the order you want. Like areas A-B-C or you could do B-A-C etc. Obviously you'd further expand with more enemies, skills, and such. You can have the typical, you make it to the king but the path is blocked or locked so you have to go down those branching areas to unlock the final door. With some crazy witch at the end of each that happens to have a key on them or however you would handle it. Maybe each new area is tied to the past iterations that felix and alice went through so you get to learn a little more about them and the past. Could even add more companions. I think there is plenty of room to expand if you wanted to.