I have to agree a bit with Mr_Gyactus, it is way too cheap to add skills from other classes. I also agree that as we level up, it quickly becomes possible to equip EVERYTHING. Once passed R15 in a class, you have more than enough points to pick and choose WS without having to worry about leaving something out.
What might be a simple solution would be to just double the slot cost of anything outside your class (go from 3 points to 6 points). Other than that, the AP cap could do with some lowering, but you would have to recalculate the distribution per level. A simple linear progression would not work that well, though, as you are extremely restricted on skills for the first couple of levels (but with how fast they fly by, it might be a non-issue currently).