Dragonblade
Adventurer
So core 'theifly' abilities might be some thing like:
"Add your class level as a bonus to hit against any foe you have combat advantage on. Additionally, you may apply one combat perk to each attack you make against this foe even if you don't know that perk, provided you meet all the perks prequisites."
But I'm pretty happy on the whole to just consider the rogue to be a particular diverse selection of mundane abilities provided that the system doesn't, as stock 3e tended to, suggest that spells are just simply better than skill.
So going back to my token idea, you could do something like "When a Rogue has combat advantage, "grit" tokens expended to boost damage do +2 per token expended instead of +1" or something like that.
Or when a Rogue has combat advantage they gain X bonus grit tokens for that round that can be spent against that foe to do things like bonus damage, forced movement, or other stunts.