First off, you'll have to give the class a token BAB, maybe based on the Wizard. That's just how d20 works.
Then you'll have to design character abilities which a) benefit the entire party without requiring the character to attack, and b) make the character interesting to play.
Recently I read a mediocre PDF which added "mythic archetypes" to d20. Maybe the less combat-oriented archetypes, notably the Oracle and the Maiden, will yield some useful ideas:
The new class could have spell-like abilities, e.g. "augury", "true seeing", or various healing spells, N times per day or based on some sort of spell points. If the class is a more efficient "healing battery" than a Cleric, with some oracular abilities, its lack of combat effectiveness might not be so bad. Other abilities to consider are "psychic" powers to read and cloud minds (think The Shadow, or "hold"/"charm" spells), or summoning critters to do the fighting.
The Maiden archetype can, with another player's permission, swap places with a stronger character who can defend her. Since you titled this thread the Bastion, we could reverse this: the character has high AC and/or Saving Throws, and can interpose himself between any other character and his/her attacker, taking the brunt of the blow. (The Armiger from Iron Heroes has this ability, or a similar one ... too lazy to look it up.)
Similarly, if the Maiden is helpless, unconscious, or dying, all characters get bonuses to their "Action Dice" (based on d20 Action Points). You could add a morale bonus to all the character's allies instead. Or maybe the circumstances are less dire: a "leader" type might inspire a morale or tactical bonus just by his presence, as long as he's still conscious and out of the fray.
The new class could have bonuses to non-combat skills, particularly social or knowledge skills. In our Midnight campaign, one character has an insanely high Diplomacy, which even after the DM's patch to Diplomacy still makes him an asset as long as he can speak to potential enemies.
Alternatively, the class could roll an extra die and add the result for all skill checks, to represent a Jack of Trades ability. The type of die would escalate based on level: d4, d6, d8, d10, d12 ...
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)