I like the new thief level, but the effect of finishing the quest does require more thought.
How about an altar to the god of thieves somewhere in the level, and you become the head of the guid by (a) killing Yergius and (b) sacrificing 6666 gold on the altar? Another option would be if Yergius had a special badge or amulet, which you can either wear to become the guildmaster or return to Twyat Pare. If you wear it even once before seeing Twyat, he refuses to take it and shakes his head sadly... 'I hoped you would resist the temptation...'
Becoming guildmaster should also have some effect for the rest of the game other than just higher prices and an achievement line. Maybe the guild level could spawn a gold pile periodically, since you get a share of what all the other thieves are taking. Maybe you can tame thieves who you meet in the dungeon just by 'C'hatting with them.
Ben