Ok, I did a cursory search and didn't find this idea anywhere in the forums. I'm sure someone is going to tell me "This was suggested 103 years ago and it was discarded..." but I wanted to make the suggestion as an idea to improve build diversity since that seems to be a very popular topic.

The Suggestion:
- Class enhancement trees should be "unlockable" for classes you don't have, just like cross-classed skills
- Exactly like cross classed-skills, these should cost 2x the AP cost
- There should be no limit on progression in a cross-classed tree, the 2x cost should easily deal with that (for instance you could never get a cross-classed capstone due to the 80AP limit)

The Reasoning:
Many builds take "dips" into other classes to get access to the low-hanging fruit in the other class trees. While this does allow for "diversity", in truth it really doesn't. What it does is incentivize people to take the capstones in Universal trees while dipping into optimal class mixes to get the lowest hanging fruit. For reference, look at all the builds attempting to maximize imbue dice. What this means in the current meta is that we end up seeing mixes of classes that offer imbue dice low in the trees. What it also means is that we see (due to the particular universal trees available to us) a higher percentage of dagger (thrown and melee), bow, and crossbow users than would ordinarily be present. By allowing people to take the capstone in their primary classes, while also making decisions about AP cost vs benefits from cross class trees you increase the number of decisions dramatically.