Haven't seen this exact idea pitched for the enhancments. This is just to throw out another options for the devs to take into consideration if total points spent can purchase in all trees for pre req.
For the core racial tree have it total points spent. You cross class (ie ranger, fighter) any poits spen in racial tree and any other tree counts towards point pre req for the racial tree. ( this one i know has been pitched)
Now the part i haven't seen discussed, or at least couldn't find any dicussion on it. What if you spent points in a class tree it counted as a point towards all of the ranger trees. example: I spend points in tempest tree and that counts as a point spent in AA and DWS tree. This will allow access to parts of what we consider core class in other trees, such as favored enemies. This will open up more AP for spending as well.
On top of this though I would say if you purchase a teir 5 in one of the trees it locks out all other teir 5 tree abilites as to not become too overpowering.
I do understand that this may not competely take care of the cross-class problems with other class abilites points spent ect.. Just a thought to see what others think and get the idea out there as an alternate to total points.