Post got eaten by the great forum purge mk 2, so since the devs are clearly still batting unlock requirement notions around:
Really, really, really, cannot vehemently disagree with that type of straight-jacketed approach enough, but if you're absolutely determined to go that route, a smoother approach might be raising the tiers from 5 to 8, and granting 1 unlock per class level.Originally Posted by Vargouille
Say, rogue 3 then gets one of:
Assassin, mechanic, or acrobat t3,
Assassin, mech or acro 2, and one tier of another one
all 3 at tier 1.
That'd put it at 15 levels for all of the current tier 5s, 18 for all of the theoretical tier 6s, 21 for tier 7s, 24 for tier 8s. Since 21 and 24 are impossible to hit barring epic levels granting actual class levels, there's your restrictions based not only on class, but also on tree tier availability, as going further up in one reduces the opportunity to go up in others in a more fine-grained manner.
tl;dr: The intent at least is to provide a more smoother tier progression, as well as a range of more interesting choices throughout more levels while providing an inherent tradeoff that isn't either too front- or back- loaded. (Hence 8, since 1 t8 would fit between the core 6 and 12, and 2 would fit between 12 and 18 descision-wise as far as required class-levels to unlock.)