@bythewell - Actually my favorite muffin is one that has been made with a lot of bananas in the batter. I like it to be sweet but not too rich. Dates are another really good thing to add, although they can be too sweet if you use too many of them.
Muffins are one of those great recipes that you can play around with, although you do have to make sure that you follow a few simple guidelines, like don't over-stir the batter. They won't rise if you beat out every single lump. But if you want to throw some random ingredients in there, go ahead.