At the moment, Poison thiefs are rewarded for exploration by discovering free (coinwise), powerful and varied effect poisons in random locations and niches in the game. Once they find one, it goes on their list and if they find the right enemy for it then they keep going for it, as they can only carry so much (and ouch on the weight of some trapper ingredients!)

Replace the idea with reagents. Give mages a transformation spell that melds up to 5 reagents into a mystical barrier. The scale would go shield - aura - shield+aura - shield+barrier - aura+barrier - abs. Duration depends on the power of a reagent, effects depend on the combination. To make things interesting, certain combinations, like two or more acid-related reagents, gives additional resistance to acid as well.

To pre-empt the cynasism, yes, we've turned mages into part-time alchemists. There's the possibility of adding a shop with a fixed level so that everyone at hero can at least buy enough reagents to get aura and shield. Mass finding/slaughter/looting of mage labs, magey hideouts and mages in various locations is now the national pass-time! To reduce the frustration of difficulty of any kind of recipe system, replace detect artifact with a similar spell that tells you the properties and strength of a reagent.

This magical reagent adds 3 to your spell power, 4 to it's duration and has cold resisting properties.

Spell power ranges go something like 1-3 = shield, 4-6 = aura... etc etc, up to barrier level which requires a reagent in the mix that's going to add a significant spell power number like 15. The idea being that you can burn more reagents to get a big duration, big power, full abs or if you feel less threatened, try ommiting a rarer reagent or two to try and get something more your current threat level.

To handle numbers of reagents involved, they're now back-packable.

Looking over it, it sounds more time consuming at first but more rewarding in the long run and the kind of knowledge that could be traded openly. A better version of this idea can involve taking the reagents and creating a wand - with charges, duration and which spells it adds (i'm assuming wands can be tooled to grant multiple spells) being part of the mix.

Make a quest that involves a lvl 35 going out to find one random basic ingredient, the quest giver hands them 2 more and success is when they make a wand and hand it back - rewarding them with 3 just like it to try out.