Multi-Dual-Omni-Classing

I've been taking the opportunity, during my break from GMing, to think about multi-classing. Or dual-classing, or whatever.

At present I'm using a sub-class system, in which a bunch of abilities is tacked on to one of the basic classes and just adds about 15% to 33% to the experience points needed to rise in level, but I'm not entirely happy with that. I'm tempted to rebuild all the sub-classes as fully-fledged classes and move to a more open-ended multi-classing system, similar to the d20 system.

My design aims are these, more or less:

1. No species-based restrictions on multi-classing (because, phooey)
2. Fairly easy prerequisites for most classes, enabling at least 1 in 3 normal characters to multiclass if they so desire
3. Simplest possible prerequisite chains (without enabling ludicrous class combinations)
4. Simplest possible level advancement requirements (i.e. experience points)
5. Maintain compatibility, as far as possible, with by-the-book 3rd-party resources (e.g. adventure modules)
6. Maintain character-level HD cut-offs (i.e. no more HD after 10 levels, regardless of how those levels are composed)
7. Character levels are counted individually by class, not together (e.g. a L3 fighter, L5 thief is NOT an 8th level character)
8. Experience is always split between all a character's classes, even if for any reason they can no longer advance in a class

At present my thoughts are somewhat inchoate, and tending towards the general.

I'm thinking (so far) that multiclassed characters would be allowed the best saving throw and attack of any of their classes, with any weapon allowed to any of their classes, but be restricted to the lightest armour of any of them to be able to get the benefit of all class abilities. For example, a 5th-level fighter, 5th-level thief would attack as a 5th-level fighter, could use a two-handed sword, but could not wear any armour heavier than studded leather, nor could he employ a shield, while doing thief things. A magic-user-thief could carry and use a longsword, and attack as a thief, but can't cast spells in any sort of armour. And so forth.

I'm also thinking about the time and training that is assumed in the background of any of the basic classes. For example, a 1st level magic-user is generally assumed to have had a fairly lengthy magical apprenticeship even before beginning their adventuring career, and a 1st level fighter is assumed to have had considerable military training. It may be that some sort "apprenticeship" period should be required before adding one's first level of another class could be allowed: often enough, that would have little impact on the game, since it could be glossed over with a simple decree by the GM that "two years pass while you do your basic training..."

It would mean that it wouldn't really be feasible to start on a multiclassed career half way through an adventure, but that's not such a bad thing in my view. I never liked the way, in D&D3e, a player could just announce that they were going to spend their experience to get a level in some class just because they happened to need access to those skills right then and there.

I guess I also need to think about how energy draining attacks should be applied against characters with two (or three, or four) classes, but I'll get the bones of the system worked out first.