starting with this:
each of the 3 tree slot will be reserverved for a single class. as we can only have 3 different classes on a character at any one point, this works out perfectly
the selections at the bottom will allow you to choose among the PrEs available to the class
in the example, Class 1 Enhancement Tree is taken up by the Fighter class. The list from the dropdown will include Stalwart Defender, Kensei, and whatever the other one that's suppose to come out
this works the same way for the other 2 trees
so if i have a character with fighter/ranger/rogue, my Class 1 Enhancement Tree could be Fighter, Class 2 being Ranger, and Class 3 being Rogue tree
the enhancements in the tree section will require you to have spent certain amount of APs thus far, does not matter which tree you have spent the APs in
the enhancements in the CORE section (bottom) requires you to have invested a specific amount of APs into the tree associated with that core.
so if i want to take the 3rd Core enhancement from the Kensei PrE, i'll have to have spent 10 points in the Kensei tree as well as having 6 levels of fighters.
now, the thing is, when you have spent the points into one PrE, you are now able to spwap over to another PrE to spend APs into that tree
so if i have spent 23 points into Kensei like this:
notice how it shows 23 AP spent in two different places? once in tree (for the sake of Core enhancement requirement) and once at the bottom near the Reset all trees (for the sake of Tree enhancement requirements)
now i can swap the PrE from the Fighter window over to Stalwart Defender, like so:
note that the AP spent in the Stalwart tree says "0 AP Spent", while the "AP spent total" at the bottom remains at 23. This means that what you spent in the other tree still still valid, all you did when you swap over to the stalwart tree was changing the list you're looking at.
What this does is basically making the Tree Enhancements more like the "general" class enhancements that we have right now. if you have the class requirement as well as the AP spent requirements, you can take them.
and since there are concerns about multiclassers cherrypicking all the extremely powerful enhancements with some 6/6/8 split. What if i say that some of the enhancements from the tree will require specific CORE enhancements?
AA Arrow of Slaying will require you to have the lvl 18 Shadow Arrows
Battle Mechanic's Weapon Attachment will require you to have the lvl 18 Infuse Armor
this way of gating will still limit the crossers' access to the more powerful enhancements, while opening up their choices for the lesser enhancements without severing their flexibility from the class investments
also, this obviously means removing the ******** 3 PrE tree limit