If you're saying that domains *will* start restricting access to spells, along the lines of 2e spheres (though no doubt with the Next "prepared-spontaneous" mechanic) you will make me a very happy man.
If you're also saying that you suspect that wizard traditions will also restrict access to spells, you will make me even happier!
Though for wizards, perhaps a shade less so. I don't see any reason why most wizards couldn't cast simple illusions; I just think that the more fancy illusion spells should be the province of specialists.
Hm, if all wizards are going to have a tradition, and traditions restrict spells, that means the end of the generalist wizard. And I, for one, shall not shed a tear at his passing.
No reason why there can't be a "generalist layer" of common spells for all wizards, and then the traditions build on top of that. Same thing would also be true with clerics and domains, though with different spells. Then you don't get into that jumping through a bunch of hoops to explain how each caster crosses specialties to pick up required spells, because some traditions/domains don't cover all the bases.
That also means, unlike 2E schools, that wizards might possibly pick up additional traditions as they level. Start with common+tradition. Every five or six levels, pick up another tradition or improve the current one. A similar thing could work with domains, where the cleric gradually picks up more of the domains relevant to their deity. This doesn't work without a common base, because then each tradition/domain is too broad to allow such expansion.