Wizards memorizing spells from ancient books and then forgetting it all of a sudden might sound cool, but it might get troublesome if you (the GM or player) are the one suddenly forgetting your spells and having to check your own books in the middle of the action.
I tried coming up with my own system many times, and I often erred on the side of free-form. Magic (and the gods, if we are talking about clerics) is capricious and unpredictable, and would-be-wizards should be prepare for that. Eventually I came to the conclusion that this could get unfair and quite annoying to the players, specially the ones who like tactical choices, planning in advance, etc.
When writing Days of the Damned, I came up with a very structured way of creating new spells. For BD&D, OTOH, there are so many good spells already that it seem a bit of a waste to redesign the whole system, specially after you beef up your fighter a little bit.
With that in mind, here is my minimalist system for wizard in BD&D.
Wizards have a number of magic points (MP) that follows a progression very similar to their HP progression, but using INT mod instead of CON mod (and +3 MP after level 9). By making MP similar to HP, you keep the flatter power curve after level 9. By making it recover at the same rate as HP, you have the different classes in the game replenishing resources at similar rates (although they may require a library, meditation, prayer, etc., depending on the wizard).
|Art by Rick Troula (from "The Displaced")|
Casting spell uses up a number of MP equal to spell level. Spell level is measured in a scale from 0 to 9. The guidelines are whatever spells you're using in this campaign, although rating something on a decimal scale comes very intuitively, with 0 being cheap tricks and 9 being spells like "wish" and "defeat anything without a saving throw" stuff (if you're using such spells). When in doubt, adding a level to the spell doubles the range, duration, or number of targets.
If you prefer to keep the spells as written, there is always a "predefined" spell to get you started.
Let's take a few examples from Labyrinth Lord, which I have at hand:
Continual Light is a level 2 spells that creates a "60 diameter sphere of light is brought into being, and is a permanent effect". It can also blind one creature if it fails a saving throw. If you cast at at level 5, it can blind 8 creatures around you instead, or light up a whole hall.
The famous Fire Ball is level 3 and causes great destruction. But at level 1 it just lights candles all across the table. Level 5 double the radius to 20' (which quadruple area, of course).
Teleport is a level 5 spell. It gets quite dangerous if you're not familiar with the destination. At level 6, you cast it as if you are one level more familiar with the destination than you actually are, and so on. Alternatively, add 2 levels to take a group of four with you - very useful for adventuring.
Although the wizard might seem to lose a little in total number of spells, he more than makes up for it in flexibility.
Now, you might argue that BD&D spells are not big on "balance", and I would agree. Ultimately, it is up to the group to decide what a 1st or 9th level spell looks like. For example one might limit spell's damage to 1d6 or 2d6 per level, always require a save, make magic more dangerous or unpredictable, etc. But once you got that going, it becomes a lot easier to play a wizard.
By the way, here is how spells work in Days of the Damned, adapted to BD&D. It bears mentioning that spells in this system are significantly weaker than their BD&D counterparts.
Level is spell level. Number is any "real" number involved in the spell: distance, number of targets, duration, area, weight, etc, but not "game" numbers such as HP, AC, HD, damage etc. Those are directly based upon spell level, one-to-one: a 3rd level spell either causes 3d6 damage, summons a 3 HD creature, add +3 to AC, an ability modifier, saving throws, etc.
For me, these numbers are actually easier to remember than abstract measurements (line of sight, medium distance, etc), and better for games that are focused on resource administration. Your tastes may be different, of course.
You start with a baseline 1st level spell and add characteristics as desired. For each trait you add, raise the spell level accordingly. To add 3d damage would add 3 levels. To target 32 people instead of 8 would add 2 levels. And so on.
Touching the target, using somatic components, specializing and a magic school, being in the right place or at the right time (the stars are right!), making the spell dangerous to the caster, spending any adequate resources, etc, lowers the spell level.
The last thing you need is the baseline spells. There is not one single "right" way to do this; it depends on the type of campaign you're running, the system you're using for fighters and thieves, etc. Some examples could look like this:
Fireball (level 1): does 1d6 damage to 1 target up to 100 feet away.
Teleport (level 1): moves one person (that is within 100 feet) up to 100 feet away.
Some spells are better described in multiple levels:
Levitate: level 1 to slow down a fall, level 2 to levitate above the ground, level 3 to float horizontally, level 4 to fly freely. Speed is the same as walking speed. This lasts for one minute.
So what about powerful spells like Wish, Shape Change, and Resurrection? In this system, those are rituals, spells that can only be cast with rare materials, when the starts are right... Finding the formula and materials is an adventure in itself.