I have only a very narrow window in time to shove this through, so it will be rather spartan.
I was thinking of porting the career system from Barbarians of Lemuria to OD&D. I'm not sure right now about the acquisition rate. I'm thinking maybe 2 at first level with a bonus based on prime requisite. Then perhaps a point for improvement every three levels, and an opportunity to add a completely new one every 5 levels.
The mechanic for it would be roll 3d6 vs the relevant attribute (roll under), based on the attempt. That way creative players can seek inventive ways to play to their characters' strengths. For example, the Huntsman career should grant some benefit in a survival situation, which may logically depend on CON, but a player could accurately argue that INT is as important. Later, when trying to stalk some prey, the same player could make further use of the career, this time relying on DEX.
The way I see it right now, each point in the career subtracts one from the roll. Unusual circumstances could force the player to roll 4d6, and keep the lower or higher of the three, depending on whether the circumstances were beneficial or detrimental, respectively.