When written well, having Twilight Sparkle either become an alicorn or be genetic royalty makes for a fantastic read. =) Stories put in this group must (obviously) prominently feature Twilight. It helps if one or both princesses are involved as well, but not completely necessary. Enjoy!
Also, stories placed here may have her become an alicorn later in the story, so...SPOILER ALERT, I suppose.
Besides the obvious group theme, I pretty much have just one guideline, and that's to sort any new stories into multiple categories if they apply to more than one! If a category does not exist that you feel should, either leave a message here (I check it often), or send me a private message.
NEW: Reorganization in progress! Check the new categories here, and be sure to leave comments on the doc!