Permitting Serpet Kingdoms and Savage Species, but expect an eyeroll with the Feral template, or Anthromorphic hippos and such. But yes, anything is legit. You can offer up crazy Char-Op munchkins, but I'm ideally looking for playable builds that won't make people groan at the thought, instead think "Oh, that's clever". I realize the subjectivity of such a criteria, feel free to ignore it.
Assume no Gestalt builds, simply because that opens the can-o-worms that screws with CR's.
As for level, Figure a build that could handle him/herself across the entire level progression. Perhaps cross section this character at 1, 5, 10, 15, 20 if that's helpful, or simply put together a level-by-level build.
...Here's what I'm trying to get at for myself when wrestling with this question: What are the sorts of things a character needs to be self-sufficent? Some things I've thought of include...
-A form of healing HP (Cure spells, Class abilities, +2 Vampiric Weapons...)
-A way to deal damage to all possible creature types (Feats, +1 Bane Weapons, Force effects...)
-Surviveability (AC, Feats, Great Saves, Immunities, teleport away...)
-Allies (Trained pets, Animal Companions, Summoned monsters...)
-An array of skills or options or spells that simulate skills (Open lock or a Knock spell or a door-busting strength score...)
...and so on.