Hello,
You're really not beginning with an easy task...
Classes are again being rebalanced on current pendragon patch (and in fact Broadsword is changing a huge amount of class defining abilities...)
Spell Handlers are still a mess, and will obviously need some complete re-work and refactoring, specially around newest classes like Heretic or Warlock which have been implemented in a horrible way !
Warlock use a lot of custom scripted code in GamePlayer class that shouldn't be here and that would need clean up if you come with a modular update relying on Game Event Listener...
You should pick up simple task to begin with and focus on classes needing minimal code change to build up your updates slowly on top of previously fixed skills.
If you work around weapon styles you should probably contact YaY, he worked on specific growth rate adjustment recently and discovered pretty interesting formula that may not be fully implemented in DOL actually.
If you still want to go through advanced spell handlers like Warlock ones, I'll need you to use new code features to prevent storing garbage values in Spell Table Columns (unlike most previously implemented spell handlers) so don't be upset if I don't merge your patches immediately and discuss a lot on code quality
If you need help to answer some questions I'll be around (here or GitHub or IRC but mostly always AFK)