Starring: 1 Evil PC and a group of Good PC's.
First some bizzare background:
Apocalyptic times coming! Signs are shown, old prophecies digged out, seers are a good career choice again. One prophecy seems to fit the situation, foretells the great Kebab beast comes to eat the world. Heroes shall come forth, the mighty trident of Quargh must be retrieved, the beast slayed, blah, blah, blah.
Of course, no one knows where that stupid trident is. Except maybe... the Pink Mages.
The Pink Mages used to be a powerful and feared group of wizards. They wore only pink, and so inevitably drifted into Evil and Madness. It took several centuries to drive them out. The few remainning Mages are now hiding (kind of hard with THAT colour), and have lost all the powerful magic, able only to scare folk away. Their former strongholds are sealed, and filled with deadly traps and strange creatures. Dark occult knowledge that mages head hurt is there, too. It may take more dungeons to explore to find enough hints...
(DM supplies the Pink Mage when he sees fit.)
The Good side:
The good guys need access into deadly places, where only a true Pink Mage may walk safely. A wicked DM may insert a line like this into the prophecy:
"And four of noble heart will save the day, and one of strange garments."
...so the lousy mage may be useful for something later. Spending so much time together means he must be kept relatively safe, and be no prisoner.
The Evil side:
The mage is a little mischievous bastard. The last thing he wants is to be dragged along with stupid paladins. As he is weak, and his spells are not really useful in combat, he cannot resist them.
On the other hand, no one wants their world destroyed. Saving the world yourself is of course stupid, but if you are forced... why not play a hero when/if it gets useful?
And, as an added bonus, he gets into places he can find power, REAL power. The places are dangerous, but strong bodyguards are around. Plenty of space for an inventive Evil wizard.
Railroading. Much pressure on both sides to not kill one another, but whether they behave nice, is left to roleplaying. The good guys can save the day, the mage can be a pest and possibly gain much power. Might work.