Think Casters should be encouraged out of the dungeons with other activities as source of progression other than defeating dungeon mobs. A spellcaster (wizard) should be able to gain progress by studying, by writing a treatise, enchanting weapons and so on and even by being part of a circle of knowledge (The erudites, for example)
Spell components are probably the best sink there is to have casters being less trivial and calculate more what they cast; specially the high end level spells which should be the most demanding in terms of components, the low end should be trivial and easy to accommodate the new player to the server. Similarly, this idea can be tied to the death system in having clerics' raise dead/resurrection be less trivial when saving someone, needing components to do so. This would certainly add to the "Witch" RP in my humble opinion, Hala witches clerics would get a nice niche of RP right there.
Components also give a dungeon a depth and a valid Roleplaying reason of repeating the dungeoning cycle; giving the PC a reason to return, because he needs a certain "X" component other than what's inside the chests of the mobs.
Gatherers would become even more of a profession in demand, miners could sink rare gems from their mining in the PC market, finding a rare component here and there when they extract the minerals from the rock. Gem deposits can be placed as well, their location based on challenge; Bluetspur would become a domain where rich gems will be found, ideal for a spellcaster (After all, why would sane men go to Bluetspur other than to be mind-raped), the depths of the mines in Dvervenheim likewise, and Hazlan. This would give wizards reason to travel and harness knowledge, power and associate themselves with gatherers to further their studies. Likewise, gatherers, fighters and rogue classes would be naturally drawn toward casters; as casters will be able to enchant their weapons, cast spell on them to help them reap rewards from dungeons and not only that, but scribe their scrolls (those handy Protections vs evil, that handy cameo/one with the land, stone skin, those see invis), enchant their jewels, and so on.
Edit: If I could, I'd even try to find a way to sink all the useless trinkets found all over the module to some merchant NPC that takes them in exchange of moderately mundane components. Perhaps CNR items like copper weapons which PC's learn from and toss to the trash after they are done, or else; perhaps taking these trinkets by weight and offering a value of "X" item in return. Find a creative way to recycle these items into something else; a barovian recycling used ore into making tools, caseroles and stuff like that. Smilarly, these useless trinkets could also be sold at the Bazaar in Dementlieu.
Along with that:
Scribing scrolls
Enchanting
Jewelry & Engineering (gem crushing & herbalism used for spell components as well)
Alternate methods of progression more so than other classes: given how much a casters' progression is tied with learning and knowledge more than physical prowess, perhaps alternate methods should be tied with their level. A wizard spending more time in a library, or in a knowledge center; should be able to gain RP XP at an accelerated rate if in presence of another character. Perhaps increase the multiplier for XP gain in certain areas that aren't dungeons.
Unique trade skill based on the caster type: Wizards enchanting, Alchemy. Offer unique possibilities that are tied with progression and strong abilities. Tie wizards' progression, for example, with enchanting his first high end weapon as a breakthrough, or scribing his first circle 8 scroll.
