I could go for an all out animals book. Huge sections of collected familiars and companions. New monster feats. Sections for playing awakened animals. And possibly playing an animal campaign in general.
Many people just min/max a Dino but GMs like mine think animals should be appropriate to the campaign setting.
I also think that beyond level 7ish growth the animal companion starts to fall behind.
It would be a splendid chance for me teamwork feats.
Although an Ultimate Party book devoted on how to get characters to work together. Crating, spells, feats, combat maneuvers etc.