My favorite new ingredient for all things of this nature, although I haven't tried them yet in Fajitas, is Jicama.
Great grilled, in a stir fry, or added to a vegetable tray along with the usual carrots, celery, etc. My wife added some to potato salad and the flavor and texture was a nice touch. No one is quite sure what they are, but after tasting generally agree that they are darn good!
When selecting one at the market, I usually take it over to the deli, and have them cut it in half. Sometimes they've started getting brown spots inside if they've been out for a while, and you'll be able to tell right away if they're good, they'll be pure white beneath the skin.
Give them a try, I suspect you'll be pleasantly surprised.