This idea hooked me. I've been pondering all day how to do this. After a pretty nasty party I finally got the thought I was waiting for.
Initially I was bothered about the +1 BAB suggested by the Legionaire class. I was thinking that it wouldn't be fair against the fighter classes to grant a +1 BAB to some non-core flavor class. Since it can be taken for a first level. Yet perhaps +1 BAB is all right? Provided the flavor class doesn't outshine the "basic" classes (Such as fighter, rogue wizard or cleric). I came up with this:
A flavor class should have as many class skills as it has skill points. Meaning that an Int 10 individual should be able to know all skills provided by the class. Example:
A Rat Catcher has a good Fort and Reflex save (but a bad will save) , 1d8 hit points, and 4 skill points. The class skills are: Animal Handling, Climb, Knowledge (Local) and Wilderness Lore.
Furthermore a favored class gains an ability equal in power to one feat. In this case Favored Enemy (Vermin). If the Rat Catcher hadn't gained two good saves maybe another ability (or feat) had been in order.
The reasoning is that if a flavor class only has a progression of one level, then it is not necessary to provide more class skills that the class has skill points. Also this helps to balance it all. It gives us the opportunity to go to town over the special abilities or to create more flavor classes and not having to restrain our creativity too much. For we wouldn't want to put the fighter or cleric out of business?
Now why should I pick a flavor class? Well, you would get flavor for your character and you would get access to a rare ability (don't let Favored Enemy (vermin) fool you. Some authors will come up with more exotic ones I assure you).
Now the big question is: Should all flavor classes grant the character the ability to count all skills of the flavor class as class skills for the rest of their career?
I say yes, if the class skills are limited to begin with.
In all modesty: I'm just trying to help.