A very different idea since folks are talking about how to blend two classes.
Beyond Character Classes is an essay I wrote about turning class abilities into feats and then giving characters two feats per level.
That essay provides more information, examples for how it could work with Fighter, Rogue, and Wizard (and one archetype for each), and a sample character to show how the end result is flavorful but not overpowered.
I really do not think Pathfinder needs more classes or archetypes. But a book about how to "multiclass" by going beyond the rigors of class and archetype could be valuable. I am sure the brains at Paizo and this forum could turn my essay's thoughts into something complete and valuable that revisits and rebalances the Core classes while adding new content for Pathfinder players would enjoy.
Thread moved to House Rules, as it doesn't have anything to do with the ACG playtest.
You might want to check out the talented classes by rogue genius games(formerly by super genius games). Its not quite classless, but they have turned now the monk, rogue, fighter, and cavalier class abilities into 'talents' and 'edges' that can be selected, with rules on mixing the monk and the rogue.
There are some drawbacks to going completely classless. Namely in terms of balance. The more flexible you make a system the harder it is to keep it balanced and make each option interesting and unique. Saga edition I think suffered from this in that often characters took a while to get going because each option had to be balanced with each other option. So you couldnt have something frontloaded like the way the monk (for instance) is structured, with a bunch of abilities at 1st level to make the basic (unarmed martial mystic) work.
You also have issues like greater potential problem interactiosn if you dont have the 'walls' of classes in place. For instance, a monk's unarmored AC bonus makes lots of sense, the concept needs to be able to work unarmored. But if a sorceror (for instance) got that bonus in addition to the defensive spells he is able to cast easily, that would be a balance issue. So it becomes harder to put in certain concepts into the game without creating additional balance problems. It isnt impossible mind you, but it becomes much more difficult.
I think you are unlikely to get what you want because of the greater emphasis has put on single classed characters throughout the lifetime of pathfinder. Like it or hate it, its pretty clear (especially with the advanced class guide) that the primary focus from the paizo dev crew is to make it so you can play the thing you want as a single character class without having to mix and match.