First, the best way to do classes in a Basic D&D hybrid is start out with a simple list pretty much keyed to the ability scores. Fighter STR, Thief DEX, you know the routine ... Dwarf CON and Elf CHA pretty much slide in there. The runty class, I think, is best reserved for little nippers with no exceptional scores - that's how I make my gnomes. So seven classes, and it's damn clear which ones you're cut out for, and if your DM is a softie they can let you swap a pair of scores to play what you want.
|The Grimdarckane Cricketeer Bladerager (tm)|
The contradiction? For some classes that people want to play, it makes no sense, because the class is tied in with a background. Why should my vanilla cleric forsake his god and become a druid? That's as outlandish as a Christian curate suddenly becoming a Buddhist lama at 7th level (oh wait). Why should a vanilla fighter suddenly discover her barbarian roots? While it's easy for a wizard to start specializing, or a fighter to join holy orders, or a rogue to pick up the lute and become a bard, some other class-based choices cut to the identity of the character.
Sure, you could require that the would-be barbarian declare a wilderness background as a plain level 1 fighter, but then that's just another option players have to keep in mind when they start, to avoid the disappointment of being trapped in a choice they made when they didn't know any better. That way lies Third Edition madness, where optimal builds are mapped out level by level from the start like some kind of retirement plan.
So another solution presents itself: to make further specialization about what you can do, not who you are. Not about being a barbarian, but about gaining a Berserk Rage feat at level 3. Not about starting as a druid, but getting closer to nature within your faith, picking up some wilderness miracles at level 3.
But then you risk losing the simplicity of class-based design when it comes to NPCs. Instead of being able to say "This is a level 9 druid" now you have to say "This is a level 9 cleric with the Animal Friend and Plant Baron and Rockslide Impresario and Storm Meister special options."
Anyway, I see two ways out of the maze:
1. Different rules for player and nonplayer characters. The PC gets more specialized by feats, the NPCs have a set of named classes that involve a regular progression of those feats. The PCs, then, start out with a simple set of choices, but then have more diverse options than the NPCs in the fullness of time. In the meantime, the DM can handle the complexity that comes from having twenty or so NPC class options.
2. Realize that players evolve over more than one character. So, give players new to gaming or to your particular system the easy, limited set of options. By the time that first character dies or they are ready to join a new campaign, they will be more able to handle a complex set of class choices.
Which one do you prefer?