What kinds of planets taking place in the MLP universe have you made? The Mane 6 could go on an adventure through space on a Spelljammer style ship and come across a planet that similar to The Dark Crystal.

Depends on how the 'verse is structured. In the case of something comparable to the model used in Arrow 18, I'd figure that there's a planet/moon orbiting the world of Equestria for each element of harmony ala the Ptolemaic model without the shells or epicycles.