For Ceviche I like red onion in there too.
Usually I'll dice up half a mango. It may sound strange but the flavors blend very well and it counter acts the lime really well.
After the fish is done in the lime juice and before serving, slice an avocado and serve with it. Don't let the avocado sit in lime juice though.
Triggers, Sea Bass, Fluke, and Weakfish (corvina) are the best fish to use for a Ceviche recipe.