There all really good ideas, but personally, depending on your skill. i would go down the route of sequences - fibonacci in nature, golden ratio, general sequence (1,2,3,4,5). It opens up all windows as AO3 marks can be easily obtained by simply painting numbers, along side the more complex sketching of shells/pinecones for fibonacci.
If you have taken the one for sorrow idea - portraits are good or the childs rhyme; one for sorrow, two for joy, three for a girl.... This could lead in to many techniques and final pieces

A.C