The skill lines are fine.
If skills had prereqs for the other skills then there would be LESS choice.
Some branches to upgrade particular skills different ways is fine, but the game would be terrable if getting boulder roll required archer tower.
And stats aren't going to add builds either. You'll just be playing the same with more or less hp, more or less armor, more or less damage.
It's mostly the lack of skills.
Like you can't really play Sedna without getting Pounce, Heal, and the 15% move speed buff. That's 11 of your points right there, or something. Sedna is pretty much balanced around those skills. It feels really lacking if you tried playing Sedna without heal, or without pounce. I could understand without pounce, but it's hard to play her as just support. It needs more actives you can choose from to actually give choice.
Like if they added a skill that's a self buff, which improves her support abilities temporarily at the cost of the buff ending if she uses and offensive skill like pounce, then she could play better as pure support, without being overpowered offensively.
Basically.. need more than 4 actives to choose form.
Oak is a better example. For me, it seems like to play Oak you need to have all of his skills. It's hard to focus with him.. his current skills need work with that. 