Guild Wars 2

Ditch initiative and add cool downs
Simple as that; remove the initiative mechanic completely and replace it with traditional cooldowns. Because this means skills can’t be used back to back anymore certain skills will get a buff to compensate. The reason for this is to keep defensive and offensive skills separate, that way you don’t accidently destroy your survivability by using one more attack than was needed.

Redesign traps
All traps need to be brought in line with ranger traps, changing from line based to ring based, and creating persistent fields in addition to their on trigger effects. Here are some example ideas.

Needle Trap becomes Stinger Trap (based on the improvised trap Caith used in Edge of Destiny): crippling the first foe/foes who enter it and creating a persistent poison field.

Tripwire becomes Razor-wire web: knocking down the first foe/foes who enter and creating a persistent crippling field.

Ambush becomes Stalker: Summoning a single NPC thief ally and creating a persistent blind field.

Add a stealth skill to every weapon set
Let me be clear; I hate MMO stealth, but the profession is already dependent on it so it clearly isn’t going anywhere. I just fell Arena Net may as well make it official. Here are some example skill changes.

Shortbow – Infiltrator’s Arrow: Now adds stealth at the end of the shadow step.
Offhand Pistol – Black Powder: Now adds stealth in addition to blinding targets.

Replace initiative with anonymity
Instead of having initiative points thieves would now have an anonymity bar. This bar slowly fells when attacking targets, and when outside of combat. Any trait currently effecting initiative would be changed to have the equivalent effect on anonymity.

Stealth and anonymity
No thief stealth skill has a duration (stealth from combo fields and traits still does however), instead draining anonymity. In this way thieves can stay in stealth for potentially longer, and chain it more easily by weapon swapping and using utility skills, but there is still a hard set stealth duration. Stealth breaks when attacking or taking damage, and all stealth skills now can be manually deactivated by way of a chain skill called Reveal. Skills like Shadow Refuge that stealth an area drain anonymity faster than single skills like Cloak and Dagger. The point of this is to both make stealth feel like it lasts longer, while at the same time creating a hard limit on how long a thief can maintain stealth throughout a battle regardless of the number of stealth skills used. It also prevents thieves from hitting once and vanishing repeatedly because eventually they will need to stand and fight to regain their anonymity.