Ideas I'd like for the thief...
A talent that lets you hide while being observed, even if it's just for a split second. And maybe with a mechanic where you can lose it (like a spell casting roll). [Because in my game it was almost impossible to ever get to backstab. Or escape from enemies.]
That's the default in the SD games I've played in or run. In melee, thieves can spend a round outside of the scrum to hide then sneak around and get in the backstab. To be fair, in most old school games, how useful the backstab rules are will depend on your GM's rulings and whether they're being reasonable or not. I have played with a GM that was basically "no backstabs other than fully dark rooms and you can't move from where you hide" which is lousy. But having watched the "gorilla walks through basketball players" video and totally missing the gorilla, I tend to give thieves the benefit of the doubt with all hell breaking loose in melee. It's not about being invisible, it's about not being noticed.
In terms of giving the thief additional skills is the default is you can do anything you can describe that isn't implausible. That's what's meant by player skill not character skill. And you only roll when there's some kind of imminent danger and time pressure. Most times, it's just narrative. Say that you're searching the floor and you find the pressure plate. Say you examine the lock and you find the poison needle.