I would try to make blocks with sizes like 3" 6" 9" 12" so that a multiple of miscellaneous blocks can fit into the blank spots. (clear as mud?)
The layout in this mystery quilt gives a better idea of what I'm trying to say I think.
http://sewnbyleila.blogspot.ca/2014/...onth-2015.html
This person gives quite a detailed account of how she accomplished hers.
http://craftyladylindsay.blogspot.ca...irt-quilt.html