King Arthur Flour has an excellent guide for baking with alternative and ancient grains. This is a great resource for swapping different flours.
Using super ripe bananas (basically brown and no longer pleasant to eat alone) yields a better banana flavor. They also add sweetness without extra sweetener.
The protein powder has a tendency to absorb any moisture available as it thickens the batter. If you have multiple trays, I recommend filling all your tins and baking at one time instead of in batches so the batter doesn’t dry out.
I’ve made these without eggs and they’re just fine. You’ll need extra liquid so another ripe banana or applesauce is a good swap. You could also try a flax egg but I’ve never made it that way.
Most of the flours i’ve used in this banana oat muffin (and its variations) have some kind of gluten. Cup for cup (<-affiliate) or Bob’s Red Mill (<-affiliate) have excellent gluten-free baking and all purpose options.
Seriously. Check the fridge. Check the pantry. Go to town on your swaps, additions, etc. Use whatever you have. Don’t go out to the store just to buy one ingredient. Pureed carrots, sweet potato or pumpkin are all great options.