Greetings returned. Occasional lurker, first time poster.
I may have an idea to help balance your wizard-specialist change. First I'll step back and make a general observation.
The rules regarding prohibited school(s) in the SRD seem too wordy by far. I believe those rules could be stated concisely as:
* Each of the eight schools of magic has a point value associated with it
describing the level of power of that school relative to the other schools.
To specialize in one school, a wizard must select a prohibited school
or schools whose total point value equals or exceeds the point value of
the chosen school. Those point values are:
school...........point value
============================
Conjuration...........6
Evocation.............6
Transmutation.........6
Abjuration............3
Enchantment...........3
Illusion..............3
Divination............2
Necromancy............2
(An analysis that lays out a case for this rule is included below, at the
end of this post.)
Using the new structure of the prohibited-school rule given above as a tool,
some options I see are:
1) One could claim that since the college in question bans necromancy, conjuration,
and evocation (a point value of 14), a wizard specializing in no more than
14 points' worth of the other schools would be somewhat balanced.
2) A clear problem with #1 is that the additional spells per level per day
become ridiculously unbalancing (at +4 spells per level per day). The original
phrasing of the extra-spell rule is:
* The specialist can prepare one additional spell (of the school selected
as a specialty) per spell level each day
Issues of balance look a lot better with this alternate phrasing:
* The specialist can prepare one additional spell (of ANY school selected
as a specialty) per spell level each day
3) If #2 is still a problem, you could back off on the points' worth of
the other schools that you allow specialization in. "No more than 8 or 9
points' worth", for example, would allow for meaningful additional specialization,
while providing for some "niche" roles within the college (since the
specializations of members of the college would have less overlap).
[This seems roughly equivalent to the "Weak Way" proposed by KaeYoss above.]
Out of curiosity, why would these schools be forbidden? Does the college
have an ethical quandary with them? If so, why?
==============================================
Analysis
Here I just plug the numbers into the equations implied by the SRD
prohibited-school rule as seen through the lens of the new phrasing above.
Abjuration: To become an abjurer, a wizard must select a prohibited school
or schools from the following choices:
(1) either Conjuration, Enchantment, Evocation, Illusion, or Transmutation; or
3 <= 6, 3, 6, 3, or 6
(2) both Divination and Necromancy.
3 <= (2+2)
Conjuration: To become a conjurer, a wizard must select a prohibited school
or schools from one of the following choices:
(1) Evocation;
6 <= 6
(2) any two of the following three schools: Abjuration, Enchantment, and Illusion;
6 <= (3+3)
(3) Transmutation, or
6 <= 6
(4) any three schools.
6 <= (2+2+3) [minimum point value possible for a choice of three schools]
Divination: To become a diviner, a wizard must select
any other single school as a prohibited school.
2 <= 2 [minimum point value possible for a choice of one school]
Enchantment: To become an enchanter, a wizard must select a prohibited school
or schools from the following choices:
(1) either Abjuration, Conjuration, Evocation, Illusion, or Transmutation; or
3 <= 3, 6, 6, 3, or 6
(2) both Divination and Necromancy.
3 <= (2+2)
Evocation: To become an evoker, a wizard must select a prohibited school
or schools from one of the following choices:
(1) Conjuration;
6 <= 6
(2) any two of the following three schools: Abjuration, Enchantment, and Illusion;
6 <= (3+3)
(3) Transmutation; or
6 <= 6
(4) any three schools.
6 <= (2+2+3) [minimum point value possible for a choice of three schools]
Illusion: To become an illusionist, a wizard must select a prohibited school
or schools from the following choices:
(1) either Abjuration, Conjuration, Enchantment, Evocation, or Transmutation; or
3 <= 3, 6, 3, 6, or 6
(2) both Divination and Necromancy.
3 <= (2+2)
Necromancy: To become a necromancer, a wizard must select
any other single school as a prohibited school.
2 <= 2 [minimum point value possible for a choice of one school]
Transmutation: To become a transmuter, a wizard must select a prohibited school
or schools from one of the following choices:
(1) Conjuration;
6 <= 6
(2) Evocation;
6 <= 6
(2) any two of the following three schools: Abjuration, Enchantment, and Illusion; or
6 <= (3+3)
(4) any three schools.
6 <= (2+2+3) [minimum point value possible for a choice of three schools]