Hmm, if a primarch, that'd be pretty cool.
No, VERY cool.
But of course, who? First, we'll red out all the obvious ones that it won't be..
Loyalists
Lion El'Jonson - asleep in Caliban
Jaghatai Khan - Missing (possibly in Commoragh)
Leman Russ - Missing (and with his armour found on a daemon world, quite naked too..)
Rogal Dorn - Dead, body kept on the Phalanx
Sanguinius - Dead, body recovered and taken back to Baal.
Ferrus Manus - Dead on Isstvan (without a head, but his hands remaining)
Roboute Guilliman - Dead-ish, in stasis on Macragge
Vulkan - Missing
Corax - Missing, heading towards the Eye of Terror
Traitors
Fulgrim - Daemon Prince of Slaanesh
Perturabo - Daemon Prince Undivided
Night Haunter - Dead by M'Shen
Angron - Daemon Prince of Khorne
Mortarion - Daemon Prince of Nurgle
Magnus The Red - Daemon Prince of Tzeentch
Horus - Dead, aboard the Vengeful Spirit, original body and clones eventually destroyed
Lorgar - Daemon Prince Undivided
Alpharius/Omegon - Dead? Not dead? Traitor? Not traitor? Who knows?!
So, we can narrow down the list of possible candidates (or at least, unaccounted-for bodies) to:
Lion El'Jonson - Asleep in Caliban. Unless the Necrons can magically kidnap him of course..
Jaghatai Khan - Missing, hinted at being enslaved by the Dark Eldar (we'll assume this is true).
Leman Russ - Missing, armour found on a daemon world.
Corax - He's off to the Eye of Terror, we know how those journeys tend to turn out..
Night Haunter - Nope. He's dead, headless, and there is no reason for the Necrons to have his body..
Alpharius/Omegon - It could be! But it won't be! Or will it? Or won't it?!
So we have our choice of two!
Out of the duo, I'll say that, simply, the most likely candidate for being there is Ferrus Manus.
Vulkan seemed to have some kind of long-term retreat planned, like Lion El'Jonson.
The fact that he left clues for the Salamanders to find him leaves me to assume that he had some grand plan in action and felt quite confident that he'd ride it out..
But Ferrus Manus? That makes sense.
After Isstvan, only a handful of Iron Hands made it off-world.
Sure, they had the bulk of the legion elsewhere, but they were travelling slowly in the warp and didn't really have much they could do about it.
As such, although there may have been later attempts at equipment/geneseed recovery we don't know about, the Iron Hands lost Ferrus' body.
Fulgrim presented Horus with Ferrus' head. That means the body was likely to have been taken somewhere as well.
Of course, with no mention of it and the planet abandoned afterwards, there is every possibility that it was picked up by the Necrons.
With the rumoured new background, this makes even more sense.
Ferrus Manus was a fraction of a living god of humanity, the Emperor, as were his brothers.
Each had unique attributes that set them apart from the others.
Ferrus not only had this great power, but he also had his metal hands after defeating Asirnoth, which we can all safely agree was some form of C'tan.
With the new background of C'tan shards could we also assume that Asirnoth could have been one of the shards of the Void Dragon?
Plenty of fluff in the HH novels has now shown the sheer power of the Void Dragons influence over the Mechanicum, eg the Akashic Reader. The combination of Necron ingenuity and warp-based power should surely be a potent mix indeed.
And Ferrus Manus, being super-wrought flesh containing immense warp power, also combined with a C'tan shard?
Yeah.
That's mind-bendingly powerful stuff.
With the Necrons apparently now trying to find their way back into flesh bodies, surely the Iron Hands are the cosmic opposite of such a plan?
What could be more likely to pique the curiosity of the Necrons (and possibly return from the dead) than an amalgamation of two fractions of two entirely different gods that both in their own way can rejuvenate and become undying?
tl;dr - Ferrus Manus might be with the Necrons in a trophy cabinet.