I've made many suggestions to make more skills and diversify them, but each build the skill trees stay relatively static.
Making more prereqs would make less diversity, not more.
I've always said since the begining that it'd be bad. The linear progression is fine. Really, there is no tree.. They, for some reason, made it where instead of clicking a level up button by the skill you click another one.
I guess to make it look like there is 30 skills instead of only 8.
What is needed is more skills, not more requirements for the few there are. Skill level 1, level 2, level 3, level 4 is only 1 skill, not 4. 