There are four classes with 3 skill lines each. What if at character creation, when a player chooses the class which determines their skill lines, players are given the option remain a Pure Class or sacrifice 1 of their three skill lines in favor of a skill line from a different class?
The 4 classes are already quite diverse and do not fall into stereotypical roles that are ubiquitous in many rpgs. With two skills set by your class, 4 classes becomes 4 classes with 40 (4*10) total starting variants, for those that choose not to remain pure With 2/3 of your base character decided by your class, Zenimax can afford an even greater degree of freedom, above the already incredible level that is offered, and each class has the majority of their basic skills in common with their fellow class members. (No classes would yield 1,320 (12*11*10) starting variants. )
There is even the precedent, though potentially extremen in an MMO, of using a bonus/penalty: Major/Minor skills in previous Elder Scrolls games (Morrowing & Oblivion). Such could be used if there were a reason to want to limit the number of people that selected this option. Ex: 5% Bonus to class skill-lines within Pure Class, ect.
Examples:
Dunmer Nightblade-
Worships Azura and wishes to embody Dusk and Dawn;
Dawn's Wrath, Shadow, and Assassination.
Argonian Nightblade-
Worships Sithis: Die silently and embrace the void:
Shadow, Siphon, and Dark Magic.
Redguard Templar- Fighter's Guild
Only those that can destroy the daedra should be allowed to traffic with them.
Aedric Spear, Dawn's Wrath, and Daedric Summoning
Altmer Dragonknight-
I am chaos controlled:
Ardent Flame, Earthen Heart, and Storm Calling.
Imperial Sorcerer-
A high priest that escaped Cyrodil's temple:
Daedric Summoning, Storm Calling, and Restoring Light.