I think Leona is an interesting case. She definitely prioritizes defensive stats, but I think you generally end up with a little bit of offense as well. With the changes to support items, you have the option of Steel Shoulderguards or Relic Shield. I'm curious what the preference is. Her abilities have no actual AD scaling, but her AP scalings don't seem to matter so much because she doesn't really function as a burst damage support / tank. The utility from her abilities allow for her to cc and duel her target with basic attacks, which benefit from AD.
Correct me if I'm wrong about this, ..
From my perspective, AP is best if your carry / team is able to burst your target down quickly and don't require drawn out fights. Otherwise, AD improves her ability to deal sustained damage through basic attacks over an extended fight.