Another ancient world would also be awesome, like ancient greek/rome world with references to the gods and the mythology or snow world with vikings.
A circus themed world, because I allways wanted a circus themed world in bk ... because no developers could make it better than rare. The only good circus world is in timesplitters 2 (free radical design ex-rare developers
) which is a fps and its sadly only a multiplayer map. I remember glover had a circus themed zone with a couple of circus themed levels but its kinda like only a texture theming, the levels had not their uniqueness because they was allways made with the same platforms blocks and textures. Rare allways make their worlds from scratch and there not a couple of them with the same theme/textures/platforms etc. because each world is big, different and allways unique.