I got to thinking about the various worlds. Given the new potion to catch folks up to level 50 so they can enter Celestia, I figured I'd list worlds here as well as side worlds and what levels I think they have. I also listed side worlds alongside worlds I think they match with relative levels. With Grizzleheim we can only go there at level 20 and, while Wintertusk is part of it (and technically doesn't have a level limit), I don't think it'd be a good idea to go there before level 50 (though I think you'll have an easier time in Dragonspyre if you go to Wintertusk first).
Wizard City 1-10
Krokotopia 11-20
Marleybone 21-30
Grizzleheim Mooshu 31-40
Wysteria Dragonspyre 41-50
Celestia 51-60
Wintertusk Zafaria 61-70
Avalon 71-80
Azteca 81-90
Khrysalis 91-100
Polaris 101-110
Thoughts on the relative levels? Also would anyone else like to see a few more side worlds? I think Mirage could work nicely as one, as could Wierwood (both of them are mentioned in Wysteria).