About cross-class:
Realistically, it makes sense. Take any occupation. You can find members of that occupation who have talents/interest/hobbies in just about any area of life.
I disagree. Those with talents/interests/hobbies are unlikely to be as trained, or capable in them as they are in their main line of work. If they were, they would likely be following that as their occupation.
I'd probably stat out as an average Lv.1 commoner, and have an interest in history. I am not trained professionally (that is, I didn't go to uni, but do devote a lot of my time to reading) so I would not have Know(History) as a class skill, even though I used all available skill points on it:4 skill points, resulting in two ranks.
Someone who has been taught professionally, has access to the best/most current works/journals/whatever and has put the same amount of effort into it (4 skill points) would be much better than myself, with a total of 4 skill points at 1st level.
I could do something to raise my skill (not ranks) in Know(history) like take a summer course at a uni or something, while still not taking it into my life fully, and this would likely give a similar bonus to Skill Focus.
So it does reflect reality that way.
Whether you find that reflection fun or not, that is a different question.
As far as fun and gaming are concerned, though, I like the 'choose your class skills' idea.
However, as pointed out above, skills wouldn't vary much between characters. Maybe it would be better to allow each character to pick a number of cross class skills to be treated as class skills:
Barbarian....1
Bard...........3
Cleric..........1
Druid..........1
Fighter........2
Monk..........2
Paladin........1
Ranger........2
Rogue.........3
Sorcerer......1
Wizard........1
That way, you have to stay witin your class range, but you get some leeway. The odd thing about this, is that the more class skills a class has, the less they need the extra class skills...
You could do it another way: Everone gets to choose two cross class skills that they can treat as class skills. Humans, instead, get three. You might, instead, decide that instead of getting an extra two or three skills, the character swaps class skills for cross class skills. This gives a little more complexity and individuality to characters: They may choose the same cross class skills to learn, but they give up different class skills, or the opposite: they gave up the same class skills, butt chose different cross class skills to learn.
Just a (few) thought(s).