(Note: I am mostly ignorant on the spells out there and metamagic tricks.)
MY assumptions about RAI. (please comment if these are inaccurate)
1) Lesser Metamagic only applies to spells that qualify (negative and positive effects) for the lesser metamagic effects.
2) Neutral metamagic only applies to spells that qualify (negative and positive effects) for the Lesser metamagic and Neutral metamagic effects.
3) Lesser Metamagic and its paired Metamagic feat can only be simultaneously used if the neutral Metamagic effect is used instead.
4) Lesser Metamagic is ineligable for metamagic reducers.
5) When Neutral metamagic is used the two base spell level modifiers are added together to form the base spell level modifier (> or = +0) of the neutral metamagic.
6) When Lesser metamagic is paired with its paired metamagic to form a neutral metamagic it becomes ineligible for metamagic reducers like Arcane Thesis (due to a spell level modification of +0 to +1. Channel Spell might or might not be an exception to this rule.)
7) Lesser Metamagic can only be applied to a lower level spell or to a spell that will be cast with a lower level spell slot
Aim Spell (AoE)
-2
AoE -> Single target
Focus Spell (AoE)
+1
Halves Area, +2DC, +2cl
or Quarters Area, +2DC, +2cl, downgrade Evasion and Mettle
- Aim Spell I do not know enough to comment on.
- Focus Spell seems to powerful. (In cramped dungeon corridors smaller area is sometimes good), maybe just +1 DC,+1cl
Minimize Spell (Variable numeric effects)
-3
minimum value on dice
Control Spell (Variable numeric effects)
+0
Half max result of dice
- Minimize is flat out overpowered. Casters are going to use this with small dice (made even smaller by Modulate Spell). 1d4 average/minimum/minimum as percent of average = 2.5/1/40%. I would increase Minimize to a -1 to keep it balanced.
- Control is balanced at +0. There is a feat that allows this in Players Guide to Ebberron for +0. But since the Cantrip Caster would be getting this as a synergy effect without a feat maybe +1 would be more fair.
Reduce Spell (range c/m/long)
-1
single target becomes touch
AoE becomes 5ft radius
Mold Spell (range c/m/long, AoE with a save)
+0
-2DC, allies unaffected
- Reduce Spell seems balanced from some low level spells I found.
- Mold Spell could cause havoc with AMF. Other than that I do not know.
Shrink Spell (finite constant duration)
-1
half duration
Stretch Spell (finite constant duration, numerical effects or hostile saves)
+0
double duration
either half numerical effects or +2 (hostile) saves
- Shrink Spell seems fine. (I do not know)
- Stretch Spell needs a higher save bonus in my opinion.
Slow Spell (any)
-2
full round casting or double casting time
Channel Spell (any)
+2
full round casting or double casting time or longer
increase caster level and caster level cap by +2to+6
- Slow Spell seems balanced for timed casting but outside of danger it is overpowered.
- Channel Spell would be used mostly for utility and buff spells. A +2 sounds right to me. (I am not truly fit to judge it tho)
Weaken Spell (variable numeric effects)
-1
half dice results
Modulate Spell (variable numeric effects)
+1
reduce dice size by 1 but add +1 to each die roll
- Weaken Spell is balanced I think
- Modulate Spell is better than Control so a +1 or +2 seems right.
Consider the following changes
Minimize [-1] + Maximize [+3] = Modulate [+2]
Weaken [-1] + Empower [+2] = Control [+1]