For the sake of this thread assume the core of a character is BAB, saving throws, ability scores, HP die, class special abilities (back stab, turn undead, lay on hands, etc) plus caster ability or not. (skip race considerations for now). Lets just call this Red Box Stuff.
When thinking along those terms, as far as character classes, all that the new D&D editions (since 1ed) have done is essentially add:

What? That sounds crazy to me.
I just want to encourage a discussion on a classless D&D based on the OGL.
I don't think classless D&D would work because it was pretty much founded on classes, but a fantasy based game using the OGL that is classless would be extremely possible and could be fun where the players do not have to worry if they have the right party make up, or who has to be the lame duck, or are they playing their character right.
What this essentially would mean, if I am understanding the OGL and its SRD, is you take the bits you want from the SRD, and add the rules for the classless ranks to it. Take say the abilities of the classes and git them in somehow as feats but have them open to any character to choose.
You will get some pretty strange combinations, and not really be able to define what a character can or cannot do, but this is only a problem if you want a rigid system that requires exacts to detail the combat or other interactions.
If you assume that under any and all circumstances a "kobold" will act in a prescribed manner, and can use abilities X, and weapons Y, or tiems Z; then the system will work fine because it has those options and doesn't need to be built to fit what the party of characters has or does not have for classes in a classless system.
Some many enjoy this being a "D&D" system, but I wouldn't; but that doesn't mean it couldn't be a good system.
So first what would you do with the class abilities and how would you let character grab these things to be able to make their own character, and would your system worry is one character able to do more than another because of "faulty player choices", that led to a character deficient in some area, or would the system say "Hey its yours to do what you want with your character and make it how you want to play it!"
So the very least you would have to do is throw out any sort of character optimization ideas and allow for crazy combinations of abilities to cast spells and use armor, while firing a bow.
I think a point system would work best, but if you also have a system built on races, you will need to assign points to the races as well to make their "package" be weighed properly with all the other abilities. Like the red-box set had elf as a class kind of.
So for an example:
-Start as a "Human" or just a being and if you don't pick a race package you default to a human.
-Start at level 1* with 1000 XP.
--This XP is the number of points you can spend to buy those abilities or races, or profeciencies, or whatever!
-As you reach a new level you can psend more XP on new things "unlocked" for the level such as racial abilities, more powerful other abilities, or upgrades to your abilites.
--XP only means to level what abilities you can buy with points and lower. Once you are level X you don't lose XP on it, or abilities you have spent XP on. So once level 2, you are level 2 and don't go backwards again. Any unspent XP is just your points to spend on buying new things and they rollover to the next levels, so you can save them if you want for a later time.
Example character and thrown out points cost.
Level 1 with 1000:
Elf package: 500
Bow: 100
Arcane magic use: 200
Light armor use: 200
What comes in the Elf package is just arbitrary for the numbering of the points cost, and some races could cost more or less depending on what they offer to the standard "nude" character race.
So this now Level 1 elf, can wear some armor, cast some magic, and use a bow and is out of points.
You can't sell back the bow later for a sword or trade it in, but you can buy sword use later.
This is how I would approach a classless system.
Seems looking below you actually had the same idea, where I saw your example, but missed it because no on else has ever agreed that XP could be used this way when I have spoke to them before. So you are on the right track already it seems and I wasted time even posting this and typing it up.
[/nothing to see here move along]