I believe the most intricate plot won't matter much to readers if they don't care about the characters, especially in a series. So I try to focus hard on making each character, whether villain or hero, have an interesting flaw that readers can relate to.
“You must accept that you might fail; then, if you do your best and still don't win, at least you can be satisfied that you've tried. If you don't accept failure as a possibility, you don't set high goals, you don't branch out, you don't try - you don't take the risk.”