I've been noodling this for a while. The basic idea is that the scientists in each category of science--Economy, Military, and Arcane Arts--generates a small bonus that cannot be achieved through other science. The obvious contenders:

Economy: Birthrate
Military: Reduced casualties and/or increased casualties on attacks
Arcane Arts: Wizard production rate and/or thief loss reduction

The bonuses would be small and would start to make a modest difference around mid-age. The benefits could also accrue as a kind of "anniversary" event, where every a scientist contributes to the bonus only when he has been in a position for a certain number of time. On this latter idea, it could be that when a scientist is promoted, he contributes a small amount to the Special Bonus, e.g.:

Recruit --> Novice: 0.1%
Novice --> Graduate: 0.5%
Graduate --> Professor: 1%

So as my province currently stands, I would have:

Birthrate: + 16%
Casualty: +/- 12.8%
Wizard production/thief loss: +/- 15.1%

As always, obviously, numbers could be tweaked to make it not OP. But it would add a little flavor and reward longevity in some way.