EDIT: The damage reduction would need to be 25% (instead of 33%) to stay in line with Weakness.
In order balance out the lack of sustain for Thieves compared to other professions (without overpowering condition Thieves), I suggest this (baseline) change that follows the concept of Deadly Arts.
Apply Debilitated for 1-second when you strike an enemy.
Decreases healing effectiveness by 33%, and reduces direct damage output to the source by 25%.
Maximum stacks: 2
The goal of this is to give the Thief more sustainability by reducing the incoming damage as well as shortening fights by reducing the enemy's ability to heal.
Yes, this is basically a combination of lesser versions of Poison and Weakness. This is following the concept that the Deadly Arts traits Deadly Ambition and Lotus Poison: to weaken opponents by poisoning them (it makes sense).
Unlike the Deadly Arts traits, however, the new Debilitated condition (or effect if desired) does no damage to the target. It also does not negate critical hits. Additionally, it stacks in duration with a maximum stack of 2. What this means is that this condition is NOT lethal, it does NOT affect endurance, it does NOT negate critical hits, and while it has no cooldown, it is limited in duration. The damage output reduction also applies only to the Thief that applied the condition, making it much less effective than Weakness. If it does take the form of a condition (instead of an effect), then it should always be applied behind both Poisoned and Weakness, meaning those would be cleared first. Poisoned and Weakness would also overrule Debilitated, so the reductions do not stack.
This would reward aggressive Thieves. By keeping pressure on the enemy, you can shorten the fight by keeping them from healing regardless of the amount of damaging Poison application you have. You would gain the equivalent of Protection (against the target only) by maintaining pressure. However, if you do not maintain pressure, the duration is low enough to not be too debilitating to the target.
Yes, at first glance, it may seem way overpowered. But perhaps that's not the case in actuality. Thieves lack the sustain that others possess (health, armor, passives, blocks, boons, etc.)...
Even if you assume maintaining 100% uptime against the target (1 hit per second), you are reducing 25% of direct damage. If you looked at it as increasing your healthpool by the equivalent amount instead, you would increase it from 11,645 (baseline) to 15488 (compared to Marauder's 17245). That's even less than the medium healthpool of 15,922. That means you could actually play Berserker to get the extra Power instead of requiring the use of Marauder just to stay alive.
The Thief still lacks heavy armor, many passives, blocks, and Protection/Resistance boons... so it's still lacking in many sustaining abilities. The only lethality increases are completely indirect by allowing a more powerful Amulet and by the easier upkeep of healing reduction.
By doing this, Anet can then deal with the issues of "Immortal Staff Daredevils" and whatever else may arise without harming the Thief itself as a whole (there are now even calls to nerf the Shortbow of all things).
The whole purpose is to encourage Thieves to be aggressive, and to allow them to do so against the excessive amount of damage being put out without having to rely 100% on evades. It shouldn't break the game any more than the addition of the Marauder amulet or the Protection boon. And it does require constant pressure to maintain.
Even if you had to shave down the numbers (healing and damage reduction to 20%) or only allow a single stack (that would be renewed with each hit), then it would still give a big incentive for Thieves to maintain high pressure on their foe. And an aggressive Thief should be a happy Thief, not necessarily a dead one.