I was thinking that 'animal form warrior' and 'nature caster' don't really fill all the core niches of the druid. So here's my short-form rough suggestions.
Shifter - Yes, the classic. The abilities it would have would power up druid forms, and grant new ones. The capstone would be the ability to turn into a very young (thus, not very big...medium size to large size...no bigger than a horse) dragon, complete with a breath weapon SLA.
Gatekeeper - I don't know if there's a PnP prestige for it, so I'm shooting blind here, but I was thinking there would be the addition of Banishment and Dismissal to the spell list, and maybe some of the stranger spells from PnP. Like 'Precipitate Planar Breach', which causes an area to get the environmental effects of another plane, 'Planar Adaptation' which temporarily makes you immune to the environmental effects of another plane, 'Sunder Planar Boundaries' which is kinda like a poor man's Gate, except incredibally risky, since it's just a hole into another plane for a couple minutes. Things from that plane can come and go, and you have no control over them at all. Hope they're friendly. (The main function, travel, would be hard to implement so I'd say ignore it, and just make it spawn creatures that are randomly ally or foe.) It could also do things like causing all your melee attacks to be Banishing, etc.
Avenger - The 'dark' druids. They've given up on the idea of true balance, outraged at the 'civilized' races treatment of nature and it's gifts. They have the ability to summon more dangerous creatures than normal (far more dangerous) such as wyverns, as well as to fill natural creatures (including nymphs, dryads, treants, dire creatures, etc) with a great and consuming rage.
Finally, there is one other idea, though it's not quite PrE material. More like Feat material. Druids of the Talenta Plains in Eberron, and of Chult in the Forgotten realms have the option to take the feat Dinosaur Companion, which replaces their wolf with a Velociraptor. So...that's exactly what I'm suggesting. And I'm sure that one would be very, very popular. Who /doesn't/ want to watch their pet Velociraptor devour some drow?