So, ever since Metal Gear Solid came out, it feels like everybody has to have stealth in their games. The idea of sneaking around pre-dates MGS by quite a bit, but the actual mechanics of it didn't really come into vogue until about that time. Then suddenly everyone has it, even in places where it obviously does not fit, and that trend continues today (Castlevania: Lords of Shadow 2 has some especially ugly, tacked on stealth-sections). There are three big problems with stealth that most games don't handle well; non-holistic implementation, a criminal overuse of "stealth-as-puzzle" and communication of status.
The first is the one we'll deal with, though I do want to say that no game has really nailed the communication aspect like Mark of the Ninja and you should all go buy it because it is very good. I always want to point out that the "stealth-as-puzzle" mechanic is why stuff like Splinter Cell is garbage because it's not so much about being sneaky in any realistic way so much as solving an elaborate puzzle (often using only trial and error) with overly severe failure penalties. Those are completely irrelevant to this post, but whenever stealth comes up I let myself bitch about those two things because.
So, non-holistic implementation, or adding in stealth without considering the big picture (i.e. the whole game). There's a lot of reasons to include stealth in a game, from a thematic niche that expands the variety of gameplay all the way down to "because it's something everybody has so why not Game X in too". It's gotten to the point where several generations of games now just adapt the same sort of stealth mechanics because they "worked" before.
I suppose we have to distinguish between stealth as a technique, not as a gameplay mechanic. The easiest way to add stealth to any game is to include a button that lets you turn invisible, but I think we can all agree that being invisible when trying to stay out of sight is like having a tactical nuke in a shooter. It's kind of like cheating and it becomes an annoying trump card specific classes get that then has to be balanced against their other abilities (i.e. spike damage), leaving them as annoying one-trick ponies built around the imbalanced mechanic that defines them.
My ideal stealth situation is one where the stealth is natural and skill-based rather than build around an artificial mechanic. Any player should theoretically have access to stealth, because staying out of sight is something anyone can theoretically do... except when the game isn't built holistically with stealth in mind (i.e. there are giant nameplates over your head so people can see you from 5 miles away.) Right off the bat, looking at the big picture, nameplates have to go. Tab-targeting has to go. Being able to call targets can remain, but it has to be location-based, so you're saying "There's a guy in this area" not "kill the thing with the giant mark on it." Basically anything that allows a player to be seen or targeted in a way that isn't based on another player's ability to discern their presence based on visual or aural evidence alone. Then, in theory, a player would be able to dye their armor green, stand in the bushes and have enemies walk right past them, or find refuge in the shadows not because they specifically use Shadow Refuge The Thief Skill™ but because other players can't see in the dark. The player has to put the work in, not just hit a button and turn invisible.
There are automatic balancing issues here that can take advantage of the fact that this is a videogame. Heavy armor reflects light more than leather, which reflects more than cloth. Dangly shiny bits aside, you're more likely to see a player in metal armor because it's shiny, even when dyed. Especially in the dark. Likewise, the sound design for metallic armor is noisy and clangy, while footfalls in leather and cloth are comparably quiet. Thus, even though heavy armor classes can hide, it is harder for them to do so, with the trade off (at least in theory) being that they can survive an open fight better.
Stealth as a game-wide option rather than a profession-specific mechanic means that it becomes an effective method for ambush, but also to avoid a fight before it begins or escape one after it has started. I'll hit those last too quickly before I dig into the first because that's going to lead into a bigger discussion, "Why play a thief if everyone can stealth?".
So, avoiding an attack is incredibly useful in PVP and WvW, especially when scouting but generally any time a huge blob of players could be just around the corner (which is to say all the time.) Likewise, getting away is useful too, but it's impossible without the invisibility button. Imagine how things would change if that were different. Places like the Ruins of Power would become places of refuge with all the obstructions, twisting underwater routes and little hidden crannies. You could out-maneuver an enemy and turn the tables on them, or separate a group as they spread out looking for you and pick them off one at a time. If you know the terrain and anything about psychology. Which is infinitely more cool but much harder to design than invisibility. Thieves already have a huge advantage in that they can usually dictate when combat begins and ends, fully embracing stealth as a skill, not a mechanic makes it a little fairer for everyone.
So where does that leave Thieves, assuming we still want to fill the rogue archetype (which is basically a fantasy RPG staple at this point)? You can still support stealth without having full-on invisibility. We mentioned being able to see in the dark earlier; improved 'natural' light radius would be useful for thieves trying to avoid detection (other classes should be able to trait into it too). Shadow-stepping feels appropriate for the profession in general; since they're meant to be stealthy, being able to jump around feels perfect, especially to turn a corner and then shadow-step the other direction, or using Shadow Return to disappear behind a pursuer. More of that is always good as mobility and evasion has always felt more appropriate to me. They're still getting the tools to dictate when combat begins or ends and on what terms (picking favorable terrain would be a huge part of playing the profession), which feels right for the thief, but it's not so simple as turning invisible and slinking away.
Stealing is still a nice mechanic, though I wish it had a few quality of life changes (a short description of what the item you've stolen does, appearing above the initiative gauge would be useful for new thieves). They could also incorporate a unique class boon or conditions to give them an edge (for example, thieves could be the only class with access to a sort of "Super Vulnerability" condition, given their archetypical abilities at finding weak spots in their victims for a quick assassination).
Mechanic-based stealth itself is not all bad either. For example, an ability that allowed you to turn transparent (not invisible) when standing still would be perfect, allowing you to stand in the shadows or dense foliage and avoid detection. Likewise, a trait that allowed you to extend the duration of this semi-invisibility to a few seconds of movement would be a nice use too. Mesmers would be great at creating decoys and then standing still to throw people off. I'd love it if Mesmers had a skill that allowed them to create a clone that ran straight ahead (pathfinding, I think, is probably too much to hope for) and caused some adverse effect if attacked. Likewise, creating a smoke screen that actually obscures players' sight of an area would be interesting, especially used at choke points and especially if the thief can use different kinds of smoke ("Is that poison gas or just harmless smoke?!" and can the thief actually turn one into the other at will?). Or double down on traps.
There's also something to be said for giving thieves a niche as the terrain-abusing profession, like giving them the ability to climb up surfaces. This will never happen in Guild Wars 2 (too easily abused, too easy to break outside the map, etc.), but it's an option that breaks the "Thieves = Invisible" approach. For a game to really take advantage of it, there'd need to be multiple layers to every area, with thieves being able to move between the regular ground and the overhead layer (darting between tree-branches, support beams, column tops or if you want to get really crazy, going all Spiderman across the ceiling.) At that point though, it's become almost like building two games, but if every profession could take advantage of it (or other 'layers' of the world) in some way it might be worth it. Still, even without going to that extreme, you can still imagine a thief scaling a wall in WvW and being able to pick the locks on the gates from within. It would create absolute chaos, but chaos is something I think WvW needs anyways (I'm of the mind that it's just too predictable).
The problem is, it's too late for these kind of changes to come to Guild Wars 2, so we're basically stuck with Cheap Invisible Thieves forever. But I still wanted to talk about it because. At least GW2 gets the pace right, a lot of MMOs build in movement-speed delays with stealth which means Thief / Rogue players have to creep along to be effective while everyone else charges in or has to wait.