Sounds like a really fun challenge rootyr. Quilt block combos with secondary surprises are my favorite - especially when they use two blocks. I can't wait to look up all the suggestions you've gotten here.
I hope when you all present your challenge quilts that you can share some of your favorite results on the QB with all of us!
It just so happens that a couple of weeks back I ran across one combo while fooling around with EQ7 and my hubby asked me to make it into a lap quilt for him.
It combines Goose in the Pond with Fifty four-forty or Fight. You get a really nice secondary circle formed. Here's a mock up so you can see what I mean: