I have two suggestions to improving XP and skill levels.

1. Allow the cost of a skill level to be a fixed amount of XP, this would let skill work more like enchantments, for example: if each skill level cost 1395 XP it would be the same as a fixed 30 levels

2. Allow traits to apply a multiplier to a particular skill level's XP cost. This would allow traits to be used like character classes. example: Two traits Warrior and Mage, Warrior would have a levelMultipler of attack|0.5, magic|2 A mage would have the reverse. Using the feature above, to level up in attack would cost a warrior half the normal amount (about 700) but to level up magic would cost double (almost 2800)

If you like the idea, I'm happy to contribute to the project, give me a chance to brush up on my java.