Having the Lich King become the leader of the Forsaken would be so ironic. I'm honestly not against it.
My first character I rolled in BC is a Forsaken Warlock, still my main to this day. But I would race-change him to Undead High Elf if that ever becomes an option, because that's my head canon for him.
I am the one who knocks ... because I need your permission to enter.
Umh the lich king is too big of a character to become a faction leader, the undead imho will end up with a council leadership to mirror the dwarfs in the alliance and if blizzard even introduce an allied race for them cold be a nathanos like reskin or maybe those Devy jones look alike we see in this xpack.
Yeah, they would be a perfect option, but also would just push the entire Forsaken into the bad guys rolle (I mean... right now the reason for that is boiled down to "Sylvanas", but if you get another Undead race, regardless if Sylvanas goes or stay, they are going to be the bad undeads)
Except they don't. Derek can't go home, won't be trusted, the water is still too deep.
Calia, Faol and Derek will be part of the new Forsaken leadership in the Horde and thus form the bridge that maintains the new peace after Sylvanas is ousted.
Voss is the only anti-Sylvanas Forsaken left on Horde side after the Desolate Council and now Zelling were killed, and she's not a leader type.
Also, if "lightforged undead" were on the Alliance side, the Horde Forsaken would become a non-entity, with no home, no leadership, and now completely disparate from the rest of the Horde. It would be vastly more interesting to have the current Forsaken as one society under Calia, with each individual needing to consider taking the Light's bargain, which maybe carries risks and quid pro quo's of its own.
DK's are an exception and are generally still pariahs and outcasts tolerated out of necessity, and more tied to the neutral Ebon Blade than to Horde or Alliance. Not unlike the Illidari.
A few dozen mercenaries aren't comparable to an entire race of undead being allowed into the Alliance.
The problem with the forsaken is that they are Sylvanas at last in game blizzard didn't build up (on horde side) an alternative leadership; while if we account the entire horde and her role as warchief they give her some inside antagonists since the end of legion cinematic (Baine and Saurfang talking at the banquet was the starting point) the forsaken didn't have any.
The only notable forsaken who has a bit of role is nathanos and voss but both lack any true leadership and charisma lorewise the forsaken will end up like the belfs comparing a giant leader like Kael to lord Who?
In a certain sense the same problem is on alliance side where both Dwarfs and Gnomes (now that mecha is an ice sculpture) both didn't had or have a good successor to their leadership.
Maybe Blizzard save building up a true sylvanas alternative for the forsaken leader for 8.2 and beyond but as it is now i don't see any beside an anonymous council made of some named character without any real buildup in game.