I think the Great Cave Offensive idea would be really cool.
So, smugglers... I'm picturing piratey lookin storks, a big, green crocodile type, and a few mebe beaver or possum/rat mammals.
Probably 50 or so mushrooms, with 5 or 10 goldens, which unlock stuff or whatnot.
Most likely 5 Overworld areas, you know, Mushroom Kingdom, Gritzy Desert, etc. Each one probably has 10 mushrooms distributed, and 1 golden. Then, after those overworld areas, there's the "Base of Operations". Each, save the Base, will probably have 4 subdivisions.