Thieves were one of the premier solo classes for sure. Backstab was pretty awesome, specially once you got to a certain point. However thief damage was kind of lackluster after the backstab so you generally had to rely on just auto attacks to take down your target once the backstab was done. Arcadia said with the new code that on hit effects will be possible, so I think that poisons could be a great thing to add to thieves. You could potentially have a few different kinds of poisons:
One that does instant set damage, and one that adds a DOT damage, would probably be the two main damage poisons. These would probably be the main two poisons used, prally mainhand the static damage and put the DoT on the offhand.
Maybe you could also add a poison that gives a chance for backstab to lethal blow if its coated on your mainhand weapon, heartseeker poison or something.
Maybe a blinding poison that lowers a mobs accuracy. You would just have to be careful about making poisons too good, as a blinding poison could potentially be something if its too good you may end up with everyone bringing a thief to their group everytime if no one else has a blind mechanic.
Maybe a thief can even coat a missile weapon with poison and you could have crippling poison that slows a mobs movement around the room, you could backstab, then switch to missile and kite.
You may have to tone down backstab a little but if you up their other damage I think it would work pretty well. the only problem with Dual wielding is how you want to incorporate backstab modifiers because backstab was already really strong, giving it another 25% damage would be pretty huge if you backstab with the offhand weapon as well. You could maybe make it so thieves can backstab with their mainhand weapon only, and the offhand just does a regular attack for 25% damage when you backstab. That way you get a little bonus damage but its not crazy.
And I don’t think thieves need to able to wield two hand weapons, as that causes the same problem as dual wield would if you allow backstab with both hands. If you can make the weapons two handable and one handable then they should be able to wield weapons that have both options, but just only be able to use it as a one hander.
I suggested in another post giving Rogues riposte along with fighters, but I think if something close to this poisons system is implemented I think they will be plenty strong. And they aren’t really designed to be a tanking class, they just offtank for a few after a backstab, and then the real tank should get it off them in theory.