Waterloo/Pinckney would be my choice as well, keep it easy to get to for us "southerners". I love going up north, but one close by would be nice.
I am of the mind, anything but the MRT/NCT loop is a good choice. That suggestion comes out every time we talk about a group hang. I like going to places I haven't been before. MRT/NCT is too much of a known spot for me.
I love the NCT, so anywhere on it would be good. There is a nice state forest campground, think it is called Fairbanks, just north of White Cloud that would be nice and secluded and a good amount of hiking available via the NCT.