I'm thinking it would be fun to do something in a 3 x 3 grid format with a color theme.
That might be a little restrictive, but then the blocks might kinda-sorta go together.
There are a gazillion blocks using the 3x3 grid.
How about purple as the primary color -
I (seriously) do like purple and turquoise, purple and lime/chartreuse green, purple and orange/scarlet/gold