Warning! This post is rather long and heavy!
TL;DR at the bottom.
I kinda wanted to comment on this but I see the only thread about it is dead and I was a bit too late to the party. But these are only my thoughts, not really backed up by anything other than my perspective and understanding. Correct me at your leisure.
“There must always be a Lich King.”
We who played the expansion remember this to be eternal proof of the fact. It echoes in the back of our minds. When Bolvar, believed to have perished at the Wrath Gate, appears on top of the platform, all marked up by the fire of the Red Dragonflight; we all gasped in awe. He was strong enough to survive the attack, strong enough to survive the fire and apparently strong enough to control the Scourge, because we haven't seen it since we left Northrend. So what's been going on up there while we battled Dragons, took on one mad Orc, then more Orcs and repelled the Burning Legion? No one knows. But it could that we will find out soon enough. I believe we're all pretty fed up with invasions and boring mass evil that tries to take over and control the world. Or are we? We know where we're going, but do we have the full view of things in our back yard?
Argus will likely be the place where we'll end the Burning Legion, using our maxed out Artifacts. The developers said we'll only use them this one time. So, they must have something else lined up for us, that doesn't require a mighty weapon? Or will there finally be peace? Unlikely. There's lots of hidden threats still slumbering on Azeroth. And what's even more unlikely, is to think that the Scourge can be controlled by sanity. Bolvar put on the cursed helmet or crown if you will and it's not really been cleaned properly. Whispers still emit like a nice looping record, driving anyone who puts it on crazy. And Bolvar has been sitting there, not as alone as we might have believed.
We've dealt with Azshara (in some way), we've dealt with Emerald Dream (in some way) - everyone more or less got what they wanted, everyone got some satisfaction. This expansion has been a hybrid of Big Threats. The Burning Legion, Naga and Bad Dreams. We've lost and perhaps will lose more, but in the end we'll have dealt such a blow to the demons that they won’t try anything again for at least a few thousand years.
It was hinted to us that the Legion would return to plague us already back in Mists of Pandaria, when Wrathion showed us a vision of things to come. It all came true. Wrathion was outraged by the unsatisfactory end in Mists of Pandaria, he vowed he'd protect Azeroth against the evil that was to come but he's actually not been very good at keeping his promise. He's not been seen since.
The Lich King is classified as a demigod, undead and in a sense unkillable due to the fact that anyone can become the Lich King, requiring only that they put on the helmet. Or that is my understanding of it. (Could I be up next, please?)
A very striking quote that more or less tells what has been happening and what might be again.
“It's begun.
Young heroes... I was once like you. You have come to this place seeking to bring judgement upon the damned. You will venture deep into forgotten lands. You will see wonders beyond imagining. But be warned. The land itself will rise up against you. Long forgotten terrors will smother your courage. Sacrifice everything as the final darkness falls... in the end, all that awaits you is death. Only then will you understand - you've been following in my footsteps all along.
So come then, you heroes! Come in all your power and glory! For in the final hour, all must serve the one... true... king.”
The one true King is neither Arthas or Bolvar, but the concept of the Lich King, which is still very much alive as it is eternal. Ner’zhul and Arthas fused, the merge making them the Lich King as we knew him in Wrath of the Lich King. The remnants that was left now corrupts Bolvar.
I’m not saying that this is it or that I believe the trademark is real, but it’s definitely not unlikely as Blizzard already is clearing up most unfinished storylines, tying the loose ends. No one believed we’d see Panda’s, no one believed we’d go back to Karazhan, people were torn between the Emerald Dream, Azshara and The Burning Legion.
Everytime Blizzard makes a move, people at first question it, threaten to unsubscribe or simply they don’t believe it at all. Though there’s barely any hints towards the Lich King coming back for a second round, it’s still very likely. It’s no secret that Arthas held back the Scourge from unleashing it’s true power, same did Ner’zhul. Why? The developers said that’s up for debate. But can such force be restricted and held back for long? I doubt it. As said before, at some point Bolvar will be corrupted as well and judging from the Death Knight artifact quests, he might already be. He definitely doesn’t sound like a nice guy just helping out and doing his part.
Furthermore, when he told Tirion he had to be forgotten, when he said to “never come back” - it sounded more like a threat than a warning. Only Death Knights would be able (has been able) to safely interact with Bolvar. I’m not too experienced when it comes to Death Knights or their story, but it’s not too big a stretch to believe some would actually serve under the “new” Lich King.
However, it sounds very much like Bolvar and the good he stood for, is no longer anywhere to be found. From the way he speaks, what he says, he has no problem (spoiler) stealing life from the living and using it to create undead beings. He appears to be a true ruler, such as Arthas, of the Undead, regarding only them as his tools to corrupt and kill and showing no mercy for the weak (doesn’t that sound familiar?). It’s been discussed that maybe he does have it all under control and knows what he’s doing, only appearing evil to properly take on his role as the Lich King and ruling the Scourge as the Lich King should. But it doesn’t flow with the current events. The Lich King has many times deceived others, so I wouldn’t be surprised that the helping hand lend to us by him will turn out to be the same hand to strangle us.
Now an entire expansion based on the Lich King is something we’ve already seen once, so I don’t think that’s something we’d see again. But on the other hand, we’ve now had two expansions with the Burning Legion, so I’m not going to say it won’t or couldn’t happen. If there was to be another expansion with the Scourge and our lovely icy friend, I believe it would be done exactly like Legion. Not pure Scourge, but something on the side, like Old Gods. We’ve after all only seen a mere fraction of them here and there. And perhaps we’ll see some in Legion?
This is of course all speculation, but it makes you think. There’s at least still two tiers left of this Legion cake, so we’re up for loads of surprises.
Also, where the heck is Wrathion? The guy with all the hints and such?
TL;DR: Saying that Lichy won't be back and it's unlikely isn't necessarily so. No one thought Illidan would be back, but look at all the copycats trying to be him.