Instead of level dipping to get a class's ability, I propose spending feats to get them. It's been done many times before:
1. Paladin's smite as a feat from Unearthed Arcana
2. Fighter feats are already class abilities
3. Rogue's sneak attack as a feat from Unearthed Arcana
4. Animal companion as a feat on Wizard's website
5. Substitution levels which switch one class's abilities for another's
I propose the following two feats (the names still need work).
First:
Base ability improvement
Benefit: You can increase your non-class specific abilities as feats. You may choose either of the following:
1. +1 BAB (never greater than current ECL).
2. +2 saving throw bonus (+2 to a single save; base save can never be greater than HD/2 rounded down +2).
Second:
Class ability as feat
Benefit: You may take class specific abilities, such as a barbarian’s rage or a paladin’s smite evil, as a feat. The prerequisites include:
1. Maximum base hit points equal to or above the maximum base hit points for the level at which the class gains the ability. For example, a 2nd level sorcerer has 12 hit points (2d6 HD). Likewise, a 1st level barbarian also has 12 hit points (1d12 HD).
2. Base saves equal to or higher than the level at which the class gains the ability.
3. BAB equal to or higher than the level at which the class gains the ability.
4. You must meet all of the base class's restrictions, such as a barbarian’s alignment or druidic armor.
5. If the class ability is related to spellcasting, you must be able to cast spells of a level equal to or higher than the maximum spell level of the spellcasting class and it must be of the appropriate magic type (arcane or divine).
Special: Prestige class abilities may also be taken, with the added restriction that you must meet all prerequisites to enter the prestige class.
The ability gained does not increase in strength. In the case of scaling class abilities, such as sneak attack or spellcasting, the feat may be taken again at a higher level. Otherwise the feat behaves exactly as the class ability.
Example: A 6th level rogue wants to gain spellcasting ability as a sorcerer. The prerequisites for spellcasting as a 1st level sorcerer include 6 base hit points, +2 base Will, and Cha 11+. If the rogue met those requirements, they could gain spellcasting ability as a 1st level sorcerer. They do not gain a familiar, bonus feats, or any other abilities other than spellcasting. If they took this feat and then later took a level of sorcerer, this feat would be lost. They could, however, increase their casting ability by taking further feats as long as they met the base sorcerer requirements each time.