On another point, it looks to me like a lot of these builds, while awesome and multipurpose, are not really druid centric. Psychic Theurge and Beholder Mage are incredible, but the objective here is to construct a sort of Druid "Gold Standard" so focusing heavily on nondruid classes seems off the mark.
It's really hard to improve on the druid 20 chassis, because most prestiges aren't worth losing wild shape/animal companion progression for, and most of the ones that are worth it are flat-out broken.
Forgive me if I speak blasphemy but I don't really see why 20 lvls worth of Wild Shape and Animal companion are necessary. The abilities themselves are important, but losing a few levels for additional features does not look as awful as people are making it out to be (especially given that the loss can be offset with items and feats).
While this power is AMAZING and ultra powerful, it is also heavily front loaded. By level 10 a druid can change 3/day, for 10 hours each, into an Animal ranging from small to large size. That list is so large that a player could choose a completely different form every time they change, and still never even cover half the possible variations, and many of its REALLY abuseable effects come from feats and PrCs. Dragon Shape, Aberrant Shape, and of course the Magical Beast and Outsider forms from Planar Shepherd. Since the Druids ability to choose a form is based on the form's size, not HD, it is entirely possible for him to go the whole game simply relying on the abilities of small - large creatures.
Add to this the fact that an Amulet of Wild Shaping (40,000gp) will make up for 4 levels of lost shaping (or the Divine Minion template), and that most Druid PrCs tend to progress and (more importantly) expand wild shaping after a few levels. The fact of the matter is, unless you are dead set on assuming Elemental forms (which ARE great), it is entirely possible for a druid to remain strong even with only 5-10 levels of Wild Shape progression.Animal Companion:
Animal companion is also awesome, no doubting that. Especially at low levels where having a furry murder wall is an invaluable asset. However, as levels progress, the quality of the companion does not really keep pace with the abilities of the party. The base animal companion doesn't have much function outside combat, and even then its combat options are mostly limited to mauling a single target. By level 15 the other Tier 1's of the party will be summoning equally dangerous creatures (via Planar Binding and Planar Ally), and these summons have casting, spell-like abilities, and native powers well beyond what a standard Animal Companion will possess.
Also consider that a single feat can make up for 3 levels of lost companion progression, and Animal Companion benefits cap at level 18. This means that you can safely lose 5 levels of AC and still end up the same as a straight druid.
A druid can lose 4 levels to PrCs with no negative impact on Wild Shaping, and 5 without a lasting effect on Animal companion. So dipping Sacred Exorcist or Contemplative is viable, and the loss from Moonspeaker or Lion of Talisid can be offset. (This is assuming lvl 20, if you are building from level 1 a slight delay in power may be seen, but hopefully the PrC in question is making up for that momentary lapse)