First off, you probably should have worded the title of your post to be for PVP. How it reads is like you are complaining about PVE and all classes are fine in PVE albeit slightly stronger or weaker. In PVP there definitely needs to be a buff of armor or decrease of damage towards players to make fights not last 2 seconds but that should be solved not by a blanket nerf but specifically a nerf in PVP
Second, you mention making runemage have cooldowns and such which is awful for how mage is currently designed; the whole draw of mage is it’s uncapped potential and countless hours needed to master it. The devs would never dare nerfing mage anymore as it would cause a whole lot of the endgame playerbase to dip on the game just like what happened with the True Affliction and Triplicity nerfs driving away 99% of the oldgame mages.
The devs are focusing on buffs rather than nerfs and that is great, the solution in PVP is to reduce mage damage against players by say 10-30% for hits but things like affliction DoT stay the same and have armor in BG equivalent to +5 armor values for the class. For mage this means that more spells have to be hit which gives opponents enough time to at least attack. This does not solve telecast or anything like that but you have to realize that dodging that telecast or simply shooting them before they reach you will stop them dead in their tracks. There are counters to mage and its called a different angle of approach. An unaware mage is a dead mage in PVP
Scoundrel a similar nerf would suffice as you say to at least eliminating one shot potential that takes minimal skill (Ranger can still one shot but you need to be hit at range for it to kill making it a skill based shot on moving targets rather than just basically a 1 shot melee).