Shifting Worlds/WMG

Anathemus will be Hellgate.
What else could be a fitting end for a saga bearing his name? We know that SDM is a prisoner of The General, who also has custody of the Black Spawn. And Hellgate's theme played during his capture. It's a good bet that SDM will somehow end up. Hellgate will rise again and, joining this power to his God Aura, turn his eternal ambition into reality and become the ultimate God to End All Gods.

Anathemus will be The General himself.
Another villain who has already become iconic, another being who wants the destruction of every world, another plausible candidate for becoming Anathemus. He might  if his plans start going awry.

Anathemus will be someone we will have never even met before.
That would be more unexpected than anything else SDM can come up with.

Anathemus will be Amadeus.
It would definitely be a huge Player Punch. The fact that Amadeus and Anathemus are pretty darn similar, and we have ourselves a possibility, albeit SDM vigorously denies it.

The Voice in the interlude of SDM's fight with the General in the Gaiden is the Dark Spore.
It's the Spore's way of turning SDM's thoughts down the darker road toward Hellgate. Given the indication that that question has been asked for quite awhile, it's even more likely that the Spore is doing this.

The Voice is someone trying to reach out to SDM.
Though it's unclear whether what happened was intended or not, it could very well be that someone is trying to push SDM toward certain thoughts--most likely toward Hellgate, though.

The Voice IS Anathemus.
Maybe each Spore carries the essence of Anathemus inside, ready to blossom in the ideal subject? Would fit well with the hypothesis of "Hellgate = Anathemus" up there.

The Black Spawns are spawns of Anathemus.
The Spawns are figments of the God of Destruction immense hatred for everything, which consumes all worlds. This is why they are named the Spawns - they are spawned by Anathemus.

The Spawns are Shards of Anathemus.
Anathemus was somehow broken to pieces, and those pieces became Spawns. Only once every Spawn of every universe is collected into one can the God of the End be reborn. This is why the General is busy cluttering Black Spawns together.

The Black Mass is related to the Black Spawn and Anathemus.
They are both black blobs with no definite shape and apparently no way to harm them, and they both have a very intimidating introduction, which makes battle against them seem hopeless. Subtheories:
 * The Black Mass is Anathemus. It must reunite with the Black Spawns to become complete.
 * The Black Mass is a (rogue?) result of the General's experiments on the Black Spawn.

Nevermind the stranded General Tech vessels, the Black Mass IS a weapon of the General.
He planted it there as a method of siege on Glacius' fleet, to try and isolate them in their corner of the galaxy. The General-tech ships stranded are accidental and unintended casualties, and again the General doesn't seem to care much when his soldiers die...