I am looking into how Druids (and by extend, Planar Shepherds and Masters of Many Shapes) can Wild Shape into Outsiders from the Outer Planes. The method I have looked into before boiled down to "Ask your DM", but are there any other methods to do this? Preferably without additional multiclassing or extensive feats, but if that's how it works I'd still like to hear it. Thanks in advance!
From Faiths of Eberron, is the only way I have found to Wild Shape into Outsiders. It limits you to Outsiders native to one specific plane. Certain planes have a good selection, notably Celestia (Celestials are very powerful for their HD total, and there are a lot of them in various splats).
If you just want to be a Druid who changes form into Outsiders, though
I.e. you aren't trying to use Divine Minion for infini-outsider, then there is a solution.
First of all, be a Half-Fiend with one level in the Savage Progression (or find some other Low-LA way to get the Outsider type, like being a Lesser Planetouched, or a Star Elf). Then, dip a level in Cleric, or qualify for the Contemplative (CDivine) prestige class. Choose the Alteration Domain(Dragonlance Campaign Setting). Take the Domain Spontaneity feat (CDivine). You may now exchange prepared Divine spells of 4th level or higher for Polymorph.
Congrats, you can be Outsiders.
Alternatively, wait until level 15 and be an Urban Druid. They get Polymorph Any Object.
A Psychoactive Skin of Proteus doesn't work as it explicitly bans outsiders.
Harder, and more cheesier alternatives include things like the Recaster (Races of Eberron), which lets you add a spell to your list of one level lower than the highest level of spell you know, and that spell can be from any class list. It requires 3rd level Arcane spells, and only advances arcane spellcasting though, so either Alternative Spell Source/Southern Magician or serious cheese would be required to get in.
The Spell Domain has the spell Greater Anyspell, but it specifies it can only be cast from the Domain Slot, not any other way - so it's essentially useless to you. Also only 5th level or worse spells, so only good if you're already an outsider.
Arcane Disciple could theoretically give you the Alteration Domain's Polymorph, but again it requires Arcane Magic and it changes those spells into Arcane Spells.
The Initiate feats might be able to help you, but they all specify 'add spells to your cleric list' so it's not exactly easy as a Druid to be having a cleric list. Rainbow Druidsnake? God knows.
Complete Champion has Domain Staffs, which are like a domain, in a staff. Cast spells from them like a runestaff, but wonkier.
The CO staple of a Drakehelm from the Explorer's Handbook (doesn't get more obscure than the Explorer's Handbook) straight up adds spells to your class list. It's a magic item. And completely ridiculous.
Complete Arcane has the draconic legacy feat which adds spells to your class list. Polymorph might be in there or arguable for one of the planar dragons or the other. You could get it through shenanigans. Probably. God, I don't know.
So yes, it's definitely possible. Easiest way is to just wait for Shapechange, the God-Spell, to come online at level 17. Or level 15 with Urban Druid and Polymorph Any Object. Before that you are going to have to pimp yourself to a god and go to Complete Divine, though, looks like.
Notably, Pathfinder has Planar Wild Shape, which basically shortcuts 99% of this effort by handing you the Outsider type on a silver planar-shaped platter.
They did away with polymorphing entirely though, in favour of set lists of abilities and enhancement bonuses to existing stats, so in the words of that famous guy, 'No dice!'.