I'm a fan of this sort of thing myself, but I don't want to go -too- crazy with field creation stuff. Mist was -almost- over-stepping it IMO.
That said, I do like the idea of people having these sorts of options to play with, so in E15 the Pledge Moves are available much earlier, opening up more access to the Burning, Swamp and Rainbow fields. Not entirely sure how easily Swamp will be to abuse, but hey. Might be worth something.
As a semi-related note, our assistant developer Inuki had a fun little idea he played around with implementing in a self-modded branch of Reborn, where every single field could be created with the combination of the move Magic Room being used while holding a certain item. So for instance the Glitch Field might be created by a Pokemon holding a Data Chip and using Magic Room. It was fun, but a bit ridiculous.