Angel of balanced Traits (or something like that ^^) - cannot get a lvl-2-trait while having... let's say half of the lvl-1-traits ^^
Idea: Most players will have one "cool" trait like Cateye or Tough as nails on vl 3 before getting anything else. In this chalenge you would have to get first more than one on lvl 1, before you are allowed to get one on lvl 2.
Angel of Bad Experience - With every lvl-up you have to chose a trait, right? So... negate the bonuses! Instead of 10 more hit points lose 5 of the maximum, instead of sight radius plus one, lose one, instead of reload 20% faster, do it 20% slower, and so on.
Idea: Why are the traits only positiv? Traits could be negative, too - and as every role player should know: negative traits will add some fun to any game ;)
And for all the lava-fans...
Angel of Fire Raiser - every explosion (barrels, weapons, monsters, bla - you know, what creates explosions *g*) will set a field with a 50%-chance into lava.
Hope you got the ideas - in spite of my bad english ;)