I'm presently allowing all the extra classes in the Advanced Fantasy book in my game. However, I want to keep the basic classes somewhat incentivized, so I've added these little sweet bonuses. They apply only to the four "basic" classes: fighter, thief, cleric, magic-user. Rangers and assassins and all those mofos are already cooler than the basic classes, so no treat for them.
-Get to roll hp twice and take the best result
-Get to specialize in melee or ranged weapons (+1 hit and damage) at 1st level
-If you have dex 13+, you get 15 bonus points to distribute among you thief skills at 1st level. (Only 1st level, this is a one time bonus.)
-If your intelligence is 13+, you get a bonus 1st level spell slot.
-If your wisdom is 13+, you get an extra 1st level spell slot. Yes, this means you can use a spell at 1st level.
Yes, this does leave low ability score characters out in the cold. I'd considered opening it up to everyone, but... nah. Life's tough. Be a fighter, you'll still get the benefit there even if you have a paltry strength score.