I'd make things theme-oriented rather than formulaic. Also, note that BAB +1 is not a useful pre-req, since even a Wizard will have that by level 2.
To enter Fighter, you need:
- Weapon Focus (any)
To enter Bard, you need:
- 3 ranks in each of two different Performs, or
- 3 ranks in each of three Knowleges, or
- Skill Focus: Perform (any), or
- Skill Focus: Knowlege (any) and Magical Aptitude.
To enter Wizard, you need:
- 4 ranks each Spellcraft and Knowlege (Arcana), or
- 3 ranks each Spellcraft, Craft (Alchemy), and Decypher Script, or
- One of those three-cantrip-per-day Feats from Complete Arcane.
To enter Sorcerer, you need:
- 4 ranks each Concentration & Use Magic Device, and the ability to speak Draconic, or
- Magical Aptitude, 4 ranks Concentration & the ability to speak Draconic, or
- One of those three-cantrip-per-day Feats from Complete Arcane.
To enter Barbarian, you need any two of the following:
- 4 ranks Survival
- Athletic
- Great Fortitude
- Run
- Toughness
To enter Rogue, you need any three of the following:
- Stealthy
- Deceitful
- Investigator
- Lightning Reflexes
- Skill Focus (Hide, Move Silently, Open Locks, Disable Device, or Search)
- 3 ranks each Hide & Move Silently
- 3 ranks each Spot & Sleight of Hand
- 3 ranks each Search & Disable Device
- 3 ranks each Bluff & Forgery
- 3 ranks each Open Lock & Listen
To enter Druid, you need 3 ranks each Handle Animal, Heal, Kn(Nature) and Survival, and any one of the following:
- Iron Will
- Track
- Self-Sufficient
- Skill Focus (Handle Animal, Heal, Kn(Nature) or Survival)
To enter Ranger, you need Track, 3 ranks Survival, and any one of the following:
- 3 ranks each Hide & Move Silently, or
- 3 ranks each Listen & Spot, or
- 3 ranks each Heal & Kn(Nature), or
- Stealthy, or
- Self-Sufficient.
To enter Cleric or Paladin, you need 4 ranks Knowledge (Religion).
To enter Monk, you need 3 ranks each Tumble, Jump & Balance, and any one of the following:
- 6 ranks each Concentration & Kn(Religion), or
- Improved Unarmed Strike, or
- Lightning Reflexes, or
- Dodge & Mobility.
... but IMHO if you're going to penalize multi-classing like this, you should give a little bonus at 1st level, based on class. (1st character level, not 1st class level if they multi-class.)
Barbarian: Toughness
Bard: 4 ranks Perform in two different instruments
Cleric: 4 ranks Kn(Religion)
Druid: 2 ranks each Survival & Kn(Nature)
Fighter: Weapon Focus (any Simple or Martial weapon)
Monk: 4 ranks Concentration
Paladin: 4 ranks Kn(Religion)
Ranger: 2 ranks each Survival and Kn(Nature)
Rogue: Skill Focus (any)
Sorcerer: 4 ranks Concentration
Wizard: 4 ranks Spellcraft
Yes, these are all things they probably would have taken anyway... but that's kinda the point. Now they can spend their 1st level Feats & Skill points on stuff to make them more individual.
Cheers, -- N