For starters, I'd like to say that I'm no uber TR that understands everything 100%. But I leveled my TR to 70, got it geared and played with it for long enough to figure out where the biggest issues are, in my opinion of course. This is focusing on the most commonly played path.
Issues with the class of trickster rogue, from an outsider's perspective
Okay, the big one 1st. We all know that TRs are very strong on bosses that take a while, and really bad on pretty much everything else unless you're endgame and understand every trick in the book.
This originates in the nature of nature of the most commonly played paragon path - MI Executioner. The last feat in the tree - Shadow of Demise does 50% of the damage you dealt as bonus piercing damage IF you break stealth with an encounter power.
So what you usually do to make most use of your Shadow of Demise (aka SoD) is enter stealth, use daily for the extra DPS you get from either Lurker's assault or Whirlwind of blades, use an encounter on your target - Lashing blade for single target, blade flurry for multi target.
That will do the initial proc for shadow of demise. Then, you have 6 seconds in which you do damage to your target, and let's say you do 100 damage in 6 seconds. After those 6 seconds end, you deal additional 50 damage. Seems good, pretty powerful, right? Well, no. The issue is, TRs don't get to use SoD that much because things usually die more quickly than it can proc the shadow of demise damage tick that does that additional 50 damage. So in a lot of cases, TRs don't get to use their most important feat.
The next issue would be - even IF they did somehow do that extra damage, they have almost no follow up for the AoE damage. Use Blade Flurry, use Smoke bomb, and then what? You can only spam BF until you run out of charges, at which point everything better be dead, or you're in a bad position.
And lastly, sustainability. The entire class mechanic is about staying invisible, but dealing damage reduces the duration you have, while using an encounter completely empties it. So they don't get to use their entire class mechanic more than once or maybe twice in a trash fight (fight against masses of enemies), so they lose out on their DPS there too. For a comparison, a class that's usually dedicated to doing AoE damage - CW, has a feat that reduces its AoE damage, but increases its single target damage - Focused Wizardry. That pretty much covers the issues CWs could have in single target, and now with loadouts, they have no issues doing their roles (AoE DPS and a lot of single target nukes).
TRs don't get that. So not only can they not do single target consistently due to how SoD works, they also get nothing for AoE damage.
This not only makes them less desirable in late game dungeon runs, it makes them not viable.
There are some other, albeit minor gripes as well, but we'll leave it at that, shall we?
But how do you fix such a specific issue with a class?
Starting from the top - SoD. Instead of it ticking additional 50% damage after 6 seconds, make everything do bonus 50% damage during those 6 seconds. So every 100 damage hit will do 100+50 damage during the SoD buff. So basically, change it from a debuff on enemies that makes them take 50% damage after a period of time, make TRs get a 6 second buff that just makes them deal 50% more damage while they have that buff. Heck, make it 60%. Rest can stay identical. Temporary buffs like that definitely aren't a problem - it would work almost exactly like the new artifact, Soul Sight Crystal works. This would make them more viable in both single target and AoE damage.
But as I said, while they can do the initial proc for damage in AoE, they have no follow up. If they can't hit anything but one enemy during that 6 seconds, it's all rendered moot. That's why - Blade Flurry . Nerf it. By 10-15%. And change it into an at-will. This will be one of the most significant changes you could do to the class, they're in dire need of an AoE clearing tool.
Not only will this fix the issue where TRs can't deal with trash at all, this will make them viable as the main DPS in any group as they'll be able to deal with it consistently, unhampered by the normal cooldown after they run out of blade flurry charges. I can guarantee you, people would use this, not because it's so great it's a must have in any build, but because they just simply don't have the AoE clearing abilities they need for late game dungeons, and this would fix it.
And now we come to the more optional, but still definitely optimal change for the class. As I said, TRs don't get to be invisible as much as they should be able to. Here's where I propose this change. The class feature, Invisible Infiltrator increases your damage by x% after using a daily power and refills your stealth meter. Add just one more thing to it. After using a daily power, you also gain stealth for doing damage for x seconds.
This is what connects everything into one place. Use daily, use encounter which empties your stealth, do some left and right clicking with your brand new blade flurry, by the end of the buff, you've got most of your stealth back, almost ready to do another rotation.
Why I proposed these changes
Now, this is an optional part of the text, don't have to read this if you don't want to. Still would be good, so you understand my reasoning.
Let's start from the bottom. If all of my suggestions turned into reality, the class would work like a clockwork. From a player's viewpoint, none of these changes would really change the feel of the class - it would still remain a Trickster rogue. You'd still thrive to have the perfect rotation, and that would still require the skill it does now, and class purists wouldn't feel like it's a different class.
From a technical standpoint, it would increase the sustainability of the entire class just through the virtue of it being capable of dealing with different kinds of threats. The damage wouldn't go up on single target, it would be just more constant, and less of a very specific situation damage dealer. The AoE damage would because they'd finally have a tool to deal with situations in which they're fighting loads of enemies. What TRs need isn't a buff, but consistency, and that is exactly what they would get.
It would be more consistent because of how SoD works. They would no longer happen to lose that extra 50% damage because they'd always get it because it applies on-hit, rather than after a period of time. So they're not getting a buff, strictly speaking. But they'd work better, and perform better. It wouldn't necessarily be easier either, you've still got to hit the timings and positioning to make use of your arsenal, and since it's not a buff, how would people perform better?
It would be better because their main ability wouldn't be made redundant by the pace at which the player is going.I forgot to add, as my main is a GWF, I am definitely not affected by these changes. I have no bias for or against this class, and I only played it for a short amount of time to see what's the fuss about. I have concluded that the changes would be very welcome, but not nearly as drastic as people may think. Straight up buffing their damage would be nowhere nearly as effective as doing the changes I proposed, and it would make the class unbalanced, once again.