The concept of 'game balance' these days is a bit foreign to me. I have always pictured campaigns as being open ended, a bit deadly and characters of all skill levels running around. A low level character stumbling into an encounter too challenging for them just made the game more 'realistic' for me.
When I was younger (like around 10 years old) and first introduced to the game, Magic-Users were always a bit tricky to figure out. They tended to play a very, very minor roll in my childhood games. Now days, I'm a bit more fascinated by them and their powerful magic.
Wizards, who start out incredibly weak at first, evolve into the mighty arcane casters we are familiar with. So in working magic-users into my campaign I wanted to keep that characteristic in place also, I've always liked the spell book for wizards; thick old tomes filled with mystic runes and characters, so that was, for sure, staying.
My house-rule magic-users are nothing ground-breaking, but I kinda like the general sketch of them. I may evolve things over time but this is where I'm at now. I've added some of the Holmes' elements to it as well.
The Magic-user begins his trade as a graduated apprentice and a number of spells that he has knowledge of from his studies. Additional spells can be studied or discovered through adventure. A Magic-user's spells are recorded in a large heavy tome . Any new spell that a magic-user wants to know has a chance of being knowable or unknowable. Any spell that is unknowable cannot be added to the Magic-users spell book.
Intelligence of MU
% Chance to know any given spell
Minimum number of initial spells known
Maximum spells known per level.
The apprentice magic-user 'graduates' with his minimum number of spells as per the rule, with one having to be Read Magic. The player can choose what those spells he/she wants (or random roll) from the first level spell list to complete the minimum. Thus, a higher intelligence apprentice graduates his education with with a larger repertoire. Of course, the apprentice can still only can memorize but a single spell per day. After he graduates, he is on his own. Only adventure and self-study will gain him additional spells.
These rules place some limitation on the magic-user's power, but there are ways to partially overcome them. One way is to have the spell written on on a magic scroll. Magic-users may make a scroll of a spell they already 'know' (have in their spellbook) at a cost of 100 gp and 1 week's work for each spell level.
The gold spent in this process counts towards experience (I'll be talking more about experience a future post).
Copying other spells from another spellbook or scroll into your spellbook: First you must cast Read Magic to be able to decipher the magical writing. Then you must spend a day studying the spell. Then at the end of the day you must make your know spell percentage roll. If you succeed, then you understand the spell and can copy it into your spellbook. The process leaves a spellbook that was copied from unharmed, but a spell successfully copied from a magic scroll disappears from the parchment.
If the check fails, the wizard cannot understand or copy the spell. He/she cannot attempt to learn or copy that spell again until another level is gained. A spell that was being copied from a scroll does not vanish from the scroll.
It is recommended that a Magic-user not bring his spellbook with him on his adventures. The book is quite heavy and very costly to replace. but it is up to the MU to decide if he would risk it being stolen or ruined. Now, if he wants to make a copy of his book that he can carry with him, he can, but it will cost him in terms of gold and time. 1000gp and 1 weeks work for each spell/level. If the spellbook is lost or destroyed the magic-user can try to recreate his original spells as per the creation spell rule below but has an additional 5% chance of success per character level.
My other addition would be that if a MU never runs across a spell he would like to have, he can research or create a new spell. The first attempt would cost the 2000gp per spell level and 1 week. After that week, we see if he can know the spell, if so, he has a 20% chance of success. If not, time and money lost. If he can know the spell but failed his success, he can try again with an additional cumulative 10% on each attempt.
I may evolve this a bit as time goes on but this is the foundation that I'm working from. I saw a really good article in Knockspell #2 by Brendan Falconer called Spell Complexity which gives the MU character odds of keeping the spell in his mind after casting. I like that as well but haven't applied it to the game yet.