But I do think it would be rather cool to give some
incentive to reaching 51. Not rearranging current skills,
but say, giving assassins something like "death kick of doom"
or whatever. If you did this with every class and worked on
balance, it would be fine.

The problem is choosing an ability for each class, considering
that the classes are fairly well-rounded as they are.

My idea of 51 - hero - is characters truly epic in scale.

This is why I loved the legacies. With some of these legacies,
you finally feel that "this is my own shtick. I'm a mighty warrior,
and I'm different from all the other mighty warriors. Tremble before
me!" I would move the legacies to 45 and 51 just to give a heroing incentive.

Anyway, maybe something similar to legacies for some of the other
classes would work. Assassins -do- get rising phoenix kick at 46, anyway.