So, I've done most of the core rules in my RPG over the last few days, or at least enough to playtest. Now EASILY the hardest parts, from easiest to hardest:
Gear
Magic
MONSTERS
Gear, OK, start with generic D&D type stuff. Not too hard.
Magic: Effeffeff. How do you come up and balance enough spells to magic spellcasters viable and then balance them?
Monsters: AHhhhhhhhhhhhh.
(Oh and XP rules, though those are easy)