ATTN: Nanaki and Gremlin Wizard

Originally Posted by Aleksandar

This leads me to believe we can make Character Decisions so that if you get one skill, you cannot get some others.

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The original devs never seemed to used this, so it may not work. But it's a shot.

They did use it, albeit only for weapon specialization skills, and only for a single tier. See here.

Doesn't seem like there's anything there preventing you from expanding the branches further like you've planned.