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Read ALL of the classes special abilities and pay VERY close attention to the mechanics of how they work and how they progress. You need to know that some abilities are just more advantagous then others before you even start. A quick example of that is regeneration. Monsters with regen are much, much harder to kill than other monsters of the same level. I think it shows how well you understand the importance of game balance that it's one of your top priorities. But this is a very large bite that you've bitten off. Where ever possible consider reflavoring and refluffing. That will help you in two ways, first some of that balance has already been considered and adjusted for, second it free's up your time and effort to work on things for which there is no equivalant. Third let your players know up front that you've done your best to balance things but it's a work in progess and if something is consistently to weak you'll work with them to improve it and if something is to strong you will take it down a notch. Players all hate to have their shiny abilities nerf'd but the long term game consequences are to important to ignore. Also pay attention to what changes have been made in the errata. A lot of times the change is a minor change in wording, but those minor changes let you know that how you word abilities is as important as the ability itself because players exploit every single loop hole.