As you might be able to tell from the contents of the gallery, nothing NSFW or too distressing. General guideline would be 'if it wouldn't be appropriate in the show itself, chances are I'm not going to do it'. Really involved romance stuff is okay-ish as long as it's between canon characters.
In the way of characters, so long as they fit the above, have a decent reference, and aren't a special snowflake OC to the level of red and black alicorn with heterochromia, I'd probably be alright with it. More complicated characters would take longer to do, obviously.
Also, no Equestria Girls stuff. I'm not good at drawing humans.
Fitting those, case-by-case basis.