I've come up with some suggestions which, in my opinion, should help these unpopular classes in Iris. And i was bored.
Knights-
Other melees with the same level of gears can do better than knights do in the categories knights are known for. Such as A.) tanking bosses - they are easily out-damaged by everyone, they cannot keep aggro B.) being efficient in pvp - lack of any threatening damage + sucks at annoying people + vulnerability to ignore defense skills = useless.
My suggestions are improve to add the same amount of magic defense to Shield Expert 2, Safeguard 2 and Improved Heavy armor to help them against spells. Increase the defense given by Shield Training and Final Protection 2. That should be good for improving knight's defense. As for offense, increase the "-eva" debuff of Torrent Slash to around 700-900 evasion. This should be great to have with all the evasion junkies running around. Increase the -phys defense debuff of Armor Destruction 2 to around -5k phys def. This will make the knights more threatening as they turn their targets into vulnerable victims for their teamates while his tanking abilities allows him to last longer.
As for aggro, maybe a big boost to swayed attention 2 should fix all of his aggro problems. I think increasing it from 18% all the way to 118% is only reasonable because of the amount of damage other classes can deal. Also, a boost from one-hand epics in the game when it comes to additional aggro could help them as well.
Guardians: The same improvements to similar Knight-skills that I mentioned above can work with Guardians too.
Priests/Sages: I think the suggestions of buffing their buffs are a good idea. They should be a little more noticed when it comes to actual stats instead of just increasing your size. :P
Boost the stats give by Potential Growth/Potential Growth 2 to +60/+90 allstats, respectively.
Improve the damage given by Colossal Capacity/Colossal Capacity 2 into damage 4%/6% damage, respectively. IMO % based is much better and probably more effective as it complements people who keep stacking damage. Everyone else can have it so I don't see any balance issues.
Like Colossal, Natural Power/Natural Power 2 should give 8%/12% defense. Again, better for people who stack defense

I'm not sure how defense works in the game, but it seems like 12% isnt really gonna be any game-changer. But it's definitely better than the current one.
Scouts: There's nothing sadder than scouts and hunters alike.

Assuming we're being realistic for them, they must be excellent marksmen and should be able to hit their targets without a problem, otherwise they have to adjust their builds more to have more hit chance/crit, which they deserve to passively have.
I think Focused sight must be changed to give +320 crit instead of the puny 66 it gives atm. Same goes for Marksmanship, boost the +65 hitchance it gives to +320. It should give scouts and hunters a bit of a break and concentrate more on other their other weaknesses when it comes to equips, instead of worrying low crit/hit.
That's all for now.