One thought on how to allow characters to really differentiate themselves:
Give players unlimited options for their characters, but only give them a few selections from those options.
As long as the options are balanced enough that the players aren’t obliged to make the same opening selections, the limited selections is what lets players individualize their characters. Too many selections from the option, and characters start blurring together.
This is one reason I do not plan to give clerics a very extensive spell list. Each cleric should be granted specific thematic powers from their deity that truly differentiates them. I need to think through how this philosophy can be applied to the other classes as well.