As readers of this blog have no doubt noticed by now, I like lots of spells with my D&D :) One of my favourite supplemental books for D&D is the AD&D 2nd edition Wizard's Spell Compendium -- a four volume set containing all wizard spells ever published by TSR from 1975 to 1997. That's a lot of spells.
I'm also a big fan of specialist wizards, and the idea of splitting spells into various schools of magic, which is pretty much essential once you're dealing with many hundreds of spells. So in my Labyrinth Lord campaign I've currently got four types of magic-user which players can choose from: mage (using the default AEC spell list), illusionist (also straight out of the AEC -- though I intend to expand their repertoire up to include 8th and 9th level spells), elementalist (using a custom spell list), and necromancer (which is a class I've put a little bit of work into, but only developed spells of the 1st and 2nd levels so far).
The approach I've been taking to specialist wizards is that they study totally separate forms of magic. So a mage cannot learn a spell from the necromancer spell list, and vice versa. Of course some spells are shared (both mages and necromancers can learn the fear spell, for example), but the crossover is minimal. This is the way specialists worked in AD&D 1st edition, if we extrapolate from the single example of the illusionist vs the standard magic-user. AD&D 2nd edition, on the other hand, vastly expanded the number of specialists, adding the necromancer, enchanter, transmuter, diviner, invoker, abjurer and conjurer. The difference in that edition though is that specialists can actually cast spells from most other schools of magic, being only barred from casting one or two "opposition" schools. While I like the expanded array of specialist wizards, to my mind this somewhat watered them down, as they can, in the end, cast mostly the same spells.
So I decided to base my Labyrinth Lord specialists more on the original AD&D example of the magic-user and the illusionist having separate spell lists. However upon adding the elementalist and the necromancer to the mix, a problem became clear. The main way in which wizards can expand their repertoire of spells is by finding scrolls as treasure. In the basic LL rules, any wizard (or elf, in fact) can learn any spell from any scroll, subject to the normal level restrictions. With the AEC (or AD&D 1st edition) you have the addition of illusionist scrolls as treasure, which are of no use to magic-users, except perhaps as a commodity to sell. Now if I add two (or more) extra types of wizard, then it ends up in the situation where a wizard finding a scroll in a treasure hoard ends up standing very little chance of actually being able to understand and cast the spell contained. This is a shame, because this should be a very exciting moment for the wizard's player -- the chance of finding a new spell!
I have thus found myself musing on ways to alleviate this problem. Two solutions have I found:
1. To allow all types of magic-user to learn spells from other spell lists, at a cost. This will work like spell research. If a magic-user finds a scroll containing a spell which is in his spell list, he can learn it as normal. However if he finds a spell not in his spell list, but which he really wants to learn, he can undergo a process of research -- searching out books on the topic, discussing with other wizards, perhaps even undertaking training of some kind. This process lasts the normal duration of spell research (one week per spell level), but costs half the normal rate (500gp per spell level). At the end the magic-user has learned to cast the spell. In this way, it's possible for magic-users to still make use of any scrolls they may find, though some will be far easier to use than others.
I've also decided that, to keep some "niche protection" for each type of specialist magic-user, they are only able to memorize at most one spell per level castable which is not on their allowed spell list. This allows each type of wizard to learn a smattering of other types of magic, and to have a few tricks up his or her sleeve, while still remaining fundamentally tied to the style of magic he or she was originally trained in.
2. To avoid the situation of an ever-expanding selection of specialist wizards, with minor schools such as dimensionalism, diabolism, abjuration, divination, and so on all having their own slightly differing spell lists, I've been musing on allowing such obscure branches of magic to be learned at any point in a magic-user's career, as kind of "prestige classes" (to use the D&D 3rd edition terminology). I was impressed by Dyson Logos' recent posts on prestige classes for old-school D&D, and have thought of using something similar. So if a magic-user wants to explore the far-out intricacies of dimensional magic, for example, he has the option of putting in the required efforts (time, money and XP, plus perhaps special adventures to retrieve tomes or find teachers) in return for gaining the ability to learn some rare spells -- essentially adding to his list of castable spells.
In combination, these two modifications to the system of strictly separated schools of magic create what seems to me to be a really nice balance. A few major types of magic exist, each with their own specialists. Specialists concentrate mostly on their own school, but are able to learn a smattering of spells from other schools, making scrolls a useful treasure to all types of magic-user. There are also minor, rarer schools of magic which are studied by only a select few, but which can be explored by a dedicated wizard.