Ideally a balanced PrC should compare to a base class by having stuff in both th "better than" and "worse than" columns.
Here are the class features of the Wizard
- d4 HD
- Poor BAB
- One Good Save
- One Bonus Feat per 5 levels
- Familiar Advancement
- Spellcasting Progression.
For designing PrCs, the game standards assume that you cannot give a class a lower HD, worse saves, or a lower BAB than the wizard already has. So in terms of stuff you can put in the "worse than" column, that leaves a designer just bonus feats, familiar advancement and Spellcasting Progression.
Familiar advancement is so minor as to be nearly trivial, so any PrC ability to compensate needs to be nearly trivial to compensate.
The issue with taking away bonus feats is that most PrCs are designed to be entered around 6th level, so often a given character isn't even losing out on the bonus feat for 3-4 levels down the road, and well, that's the lifespan of like half of all D&D games, so if a PrC gives a wizard anything at all now for not getting a feat later, it's usually a good trade.
So really the only thing a designer can balance a wizard PrC around is reducing spellcasting progression.
The design challenge here is that spellcasting progression includes a bunch of sub-features, and they scale in non obvious ways.
+1 caster level gives a character all of the following:
- Increased chance of getting through SR
- Increased spell ranges
- Increased spell damages
- meeting caster level prereqs for item creation
- Increased spells per day
- including access to higher level spell slots
- Increased spells in spellbook (or spells known for Sorcs,et al)
While it wouldn't be too hard nor necessarily unbalanced to design PrCs who traded features from the first four of those around for other abilities, it's the final three that cause any loss of caster level to be an opportunity cost which increases with level.
A character who PrCs out of Wizard at 6th level and into Eldritch Knight something that doesn't give a caster level effectively gives up a 2nd level spell slot and a 3rd level spell slot by taking Eldritch Knight lvl 1 instead of Wizard lvl 6. However, the eldritch knight always stays a caster level behind, so at 7th level the EK has given up a 1st level spell and a 4th level spell (plus potential bonus 4th level spells due to Int) relative to the wizard), at 8th level the EK has given up a 3rd level and a 4th level spell slot, at 9th level they given up a 2nd lvl and a 5th lvl slot (plus potential bonus 5th level spells for high Int)
So in order to just be an even trade for the spell slots part of losing a caster level, a PrC level without a caster level increase has to offer an ability which is as good as casting the highest level spell plus at least one more spell that a wizard of the same total character could cast. In order to be an overall even trade, it has to offer that plus abilities that make up for missing out on scaling ranges, damages and against SR.
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