i have a mixed feeling on this topic.. at first i thought that id love to see kil'jaeden again.. but then i realized that he whould be tuned for lfr aswell.. and honestly.. i dont want my percious kil'jaeden fight to be completly dumbed down in an lfr difficully. nor as a normal mode for that matter...
Thou if he could be like a heroic only boss.. then yeah, bring back kil'jaeden and archimonde as long as the new version matches the old version in difficulty im fine (because to me, a fight can really only be fun if its hard.. i feel next to none enjoyment in getting a boss down on first pull, it's the challenge that i like!)
There's a couple of bosses I'd like to see back, I'm not afraid to admit that I actually like seeing old faces again, loved Nefarian and Ragnaros' return.
I'd like to see Lord Kazzak and Mannoroth back first and foremost, Kazzak was a functional world boss in that he had a 'anti-too-many-people' system, and while Mannoroth haven't been a raid boss he is one of the most badass and epic things Warcraft has had, and he needs a return, simply because that ''dungeon'' called Well of Eternity didn't really do anything justice in there except for Illidan.
Believe me or not, but I'd like to see a REAL return of Kel'thuzad. Not a copy-paste with new shitty voice return, but something that makes sense both lorewise and gameplay wise. A new encounter, good lore to back it up, stuff like that. It actually wouldn't be too far fetched to see him return in a Legion based expansion. Firstly, we never fucked over his phylactery, we just left it chilling. Secondly, he was the one who summoned Archimonde to Azeroth, The Legion would benefit big time having Kel'thuzad with them back again, he is after all most likely still in possession of Medivh's Book which he used to summon Archimonde. Heck, I'm just dreaming here, he was my favorite Warcraft character and was vandalized like everything else in Wrath, what an unjust end.