I am new to this swap too, but from what I see in rules and discussion, asking for a color (only black, etc) is okay but the block choice is up to the swap partner (technically sun bonnet sue is a block pattern). That said, I doubt any partner so loves sunny sue that she will insist on making your block that pattern if you mention you don't like it! As to fabric working best, I think almost any will work fine if you have an idea of what you want to do with blocks incorporating it. I have seen single tones, batiks, great prints, solids in albums of blocks done in previous swaps. Can't wait to see what you pick!