You do have to be familiar with them though, so this is DM dependent. High HD animals might be very rare so this may get shot down.
Wild shape explicitly cues off druid level. Boosting HD doesn't automatically boost it (though inspire greatness is worth remembering for the polymorph cap).2. RAW, anything that raises your HD would accomplish that. Bard'a inspire greatness ability comes to mind, but that's only 2HD bonus. I'm sure there are others, and I'm sure you'll be inundated with suggestions shortly, but basically, anything that buffs HD would work (albeit only for the duration of the effect).