I posted my basic muffin recipe here. Here's a list of ideas to mix and match from. These are from my brainstorming and have not necessarily been tested.
Extracts: Vanilla Extract, Almond Extract, Lemon Extract, or other extracts you can find. Use 1 tsp in general, but you could increase the total extracts to 1TB for very decadent recipes.
Spices: For sweet muffins, common spices include: cinnamon, cardamom, ground ginger, allspice, nutmeg, cloves, pumpkin pie spice. The amount varies -- up to 2 tsp, but for strong spices (nutmeg, cloves, allspice) perhaps only 1/2 tsp.
For savory muffins, consider cayenne and other peppers, minced jalapeno, oregano, basil, dill, rosemary.
Zests: Lemon, lime, orange, tangerine, etc would all work. Up to 1 TB.
Moist Mixins: Any kind of berries, cherries, cranberries, apples, mango, grated carrots, grated zuchini, corn. Make sure the pieces are small.
Dry Mixins: Dried fruit (apples, pears, cranberries, etc.), raisins, dates, nuts (walnuts, pecans, almonds) -- toasted nuts add a nice depth, shredded coconut, chocolate chips, grated cheese (add to dry ingredients).
Flour Substitutes: Replace a 1/2 c flour with a 1/2 c of cornmeal, ground oatmeal, almond flour, wholewheat flour, proteen powder, or besan flour, etc.
Oil Substitutes: Instead of oil use melted butter, melted lard (savory only), almond oil, walnut oil, coconut oil, etc.
Sugar Substitues: Instead of white sugar, use brown sugar or (add to wet) molasses, honey, rice syrup, concetrated fruit juice (apple or orange). For the molasses, start with only substituting 2TB as it is a very strong flavor.
Yogurt: Use flavored yogurt instead of plan. This will make the muffin sweeter unless you decrease the sugar. Or use coconut milk or buttermilk.