Since I've played DL one thing I've loved of it is the skill tree.
Implementing one in PL would need some serious thinking. Instead of finding a very complicated method to implement it, let's do it with what we already have.
Take as an example a mage.
Your class will present, on your way to level 24, a series of skills made for mages. What happens when you put Strenght in your mage? Simply you have better Health, armour and H/S and the ability to wear bear equipment.
What I am suggesting is that depending on your class and the stats you put on your toon you will be presented with the choice to level 2 types of spells: one that your class present you or the one that your second stat is suggesting you.
Obiouvsly there must be some limitations, in order to prevent OP builds to arise.
One limitation is to have a stat requirements. For example if you want that your first spell for your secondary stat to be presented, taking the example of STR as a secondary stat, would be f.e. 20 STR.
Another limitation would be type: imagine a bear with rage and focus = unstoppable killing machine. There could be a way, from the devs perspective, where a skill can have a role (debuff I level, buff...) and if you have already a spell with that role in your spell book, your secondary stat would not present its spell.
As you have seen in my exaple of roles I have said debuff I level, I have said so because certain classes have multiple spells that have the same role (BOV & BOM for mages), this way the basic infrastructure of the class would retain the same types of spells in the end.
In my eyes, if you choose one of the 2 spells (class spell, secondary stat spell), you can't choose the one that you left behind later on, this way you have to choose one and keep the exploitation to a minimum.
Again, I am taking into consideration only two-stats and not three-stats build.