This is looking pretty nice so far! The combining of tilesets is popular and if done right as in this case it can create quite aesthetically appealing levels. I do urge you though to be careful of implementing some elements as the palettes of some objects may be difficult to distinguish from the palette of the background tiles. That is something that may arise from the looks of things (not saying this will inevitably happen however). I also would like to suggest that changing the palettes of the sprites and the status bar would help make those particular elements more fitting. The level design itself does not seem to be bad though don't let complex tileset combinations distract you from the actual level design.
wow. from the latest image, i can tell that this is probably going to be one of many great levels for the 2015 vanilla contest! the palettes really look interesting, and the mixed foreground with the castle and grass looks really interesting. the palette of the coin question blocks and the dragon coins could use a bit more work though. try darkening them so they fit a bit more with the surroundings; same with the sprites as Everest mentioned.
i always wondered why these existed in the original super mario world: invisible used blocks near ledges.
by the way, this isn't a problem since it occurred in the original super mario world, but why are there invisible used blocks all over the corner ledges?