I've decided to put what I've worked out (so far) for the Magic-User spells up front, followed by the line of reasoning that lead me to those decisions. Thus, you can skip the lengthy discussion afterwards, if not interested.
We can start with this list of spell effect categories, modified from the build-a-spell list:
- Level 1: mind, detect, protect, restrict, scale
- Level 2: movement, control, combat ability
- Level 3: dispel, cause or avoid (ranged) damage
- Level 4: instant movement, reshape, create
- Level 5: soul, conjure/summon, transmute
- Level 6: large-scale physical changes, cause or cheat death
- Spell fits two categories of the same level;
- Fits three or more categories;
- Adds an additional effect;
- Removes a limitation or otherwise improves an existing spell.
Examples: Lightning Bolt is a 3rd level (cause ranged damage) spell, so the length of the bolt is 6"; Fireball is also 3rd level but creates a three-dimensional burst, so the radius is 3".
- 1d6 if incidental (Wall of Fire/Ice)
- 1d6/spell level if limited to nearby targets (1" range)
- 1d6/caster level if ranged (Fireball)
Range (the maximum distance at which a magical effect can be created) is 12" for most spells, doubled for ranged combat spells, halved for detection, creation and transformation spells. Some utility spells that affect an object or that improve non-combat abilities like movement or senses have a 1" range. Spells that allow control of a dynamic or follow-up effect after casting (Levitation, Wizard Eye, Dimension Door) have a limit in addition to a range: 24" for most, 36" for 5th level, 48" for 6th level.
Eventually, I'll re-arrange and condense this information into some kind of table. The intention is that the GM uses this to build a small list of available spells or determine levels of spells being researched. Of course, the GM can also snag one of the existing spell lists from a clone and drop it right in without much trouble.
The categories were the result of analyzing when spell effects first appear. In the LBBs, there aren't any damage spells below 3rd level, so that's where I put the damage effect category. I restrict it to ranged damage to allow for later additions like Burning Hands. This does make Magic Missile seem out of place, unless we assume that it magically fires a real, physical arrow or quarrel, which the MU must purchase or find.
When an official spell is listed at a higher level than it should be, I looked for possible lower-level spells that the higher level spell upgrades. Fly adds forward movement to Levitate, for example. Eventually, some upgrade patterns emerged, which is how I came up with the list of conditions that add +1 level.
Some miscellaneous quantities in the spell descriptions seemed to be a multiple of the spell level, such as the 6" Lightning Bolt, or the effects on creatures of specific HD value (4th level Wall of Fire/Ice stops creatures of less than 4 HD, 4th level Charm Monster affects multiple targets if the creatures are less than 4 HD.) Other quantities, like the size of a Fireball's burst, were close enough that changing them to a multiple of spell level seemed simpler.
Ranges and durations in the original rules seemed all over the place but there were a couple broad categories. Outliers are simply changed to fit the new model: Confusion's duration is shortened, Geas's range is increased dramatically. Separating range and limit helps make sense of Dimension Door, Wizard Eye, and Levitation: they now have short ranges but high limits.
- Sealing a door (Hold Portal) is an obstacle (restrict), so it's level 1;
- Wizard Lock is better than Hold Portal (+1 level upgrade,) so it's level 2;
- Knock opens even a magically-sealed (Wizard Locked) portal, so it's also level 2;
- creating a temporary obstacle (Wall of Ice) or transforming something into an obstacle (Plant Growth) would be level 4;
- creating an obstacle that is harder or impossible to pass through (Wall of Stone, +1 level upgrade) is level 5.