The one bit of magic that really seems to be missing is life leech. You can heal, or you can cause damage, but not both at once. What I'm looking for is a single spell that does both.
My proposed solution is spell adding - that is, creating a spell using the rules laid out in the player's companion, then adding a second spell using the same rules, and the total point value of the two make the spell level. For instance, using the normal Heal Light Wounds and the reverse, Cause Light Wounds, would result in a level 2 spell, where the caster causes 1d6+1 damage to someone, healing himself the same amount as the damage he did, or reversed, the spell would allow the caster to drain life from himself to send it to an ally. Other spells include:
An illusion (visual, terrain only) and transmogrification (stones to gaseous form) to make a patch of ground temporarily vanish, though seem as if it hadn't;
Blast (instant, area of effect, cold damage) with an enchantment (hold 1 turn) to freeze a group of creatures solid;
Blast (instant, area of effect, distance 0') with movement (instant, teleport 360') to make an exploding teleport, damaging nearby enemies and getting away at the same time.
Does this make sense within the spell creation rules?
I suspect if you do exactly what you describe, and combine the effects of two spells and costing them appropriately, you should have relatively even powered spells, possibly a bit weak if the situations warranted the use of the combined effects aren't present.
I think it does make sense, and some spells have been built similarly.
However, be cautious about adding the cost of spell effects when the secondary effect is actually a limit on the first effect.
For example, it makes sense to add two effects for a spell that deals 1d6+1 damage to an enemy and heals an ally the same amount; that spell should be higher level than Cure Light Wounds. A spell that deals 1d6+1 damage to an ally in order to heal an ally the same amount is less powerful, however.
I suppose that naturally leads to the next question: backfiring or self-damaging spells. What would be a good multiplier for a spell that does some damage to the caster, as well as to the target? Say a death spell that does Nd10 damage, but reflects Nd4 damage back on the caster, or as previously mentioned, a hurt-yourself-to-heal-another type spell?
As far as adding negative effects as a limit, I usually eyeball a gut adjustment, but subtracting around 50-75% of the cost of the negative effect is a good guideline in my opinion. You don’t want to subtract the full cost of the negative effect, because the caster is always the one choosing for this effect to happen, and that inherently means that it will be less harmful to him than it would be if someone else cast it.
Presumably, for example in your ‘harm self to heal other’ spell, the caster is only casting it when they can spare the hit points, or at least at a time when they believe that spending the hit points is the right decision. This means that it’s less likely to be devastating or fatal than if an enemy cast it, and thus, less points.
(Note: I am not an Autarch, the previous is just my opinion!)