With the available gear and viability of an assassin being melee, whether it is through the martial arts tree or through Whirlwind from Chaos runeword, the main hinderence is attack rating. My thought is to add + Attack Rating (not %) either to fade and burst of speed per level (for instance +10 attack rating for lvl 1 and + 200 AR for lvl 20) or the same for claw mastery or venom. I don't suggest claw mastery as there are some sin melee builds that don't use claws.

Based on my calculations, adding anywhere between 10-25 Attack rating per level on any the above skills would be enough to make a decent amount of difference but not enough to not have to gear your character around adding at least some attack rating.