Speaking of [Laughing Ponies], I hope they don't try to sanitize Molag Bal first and general lore second. I mean, and I know you aren't supposed to do this but I have to for clarity, he is the King of R-A-P-E. That is a major thing in terms of his function as a Daedra prince.
So I assume that it'll be rated M.
Not just surprise six in the literal sense alone though.
Defilement, subversion, domination and all the less literal meanings of the word are in there too.
I can see it fitting his sphere. If he had a plan for it to work that is consistent with current lore on the topic, the most important barrier in his way is the Daedra pact with Sotha Sil by which Daedra lords only heed the summons of powerful wizards or witch covens on auspicious days.
There is also the (largely apocryphal, though not completely so) lore of the towers that hold the world up and protect it from Oblivion.
If the Lord of [laughing ponies] finds a way to circumvent these things, possibly by a subversion of the literal rules, hes not invading but swallowing, or allowing Nirn to drift into Coldharbour by something mortals did, then that would be right up his ally.
I think.
Edit: Another possible barrier for Molag Bal is the Triune.
If Dagoth Ur still sleeps under Red Mountain at the time this is set, theyll be at the height of their power.
If its set after he has awoken he will have done so recently and the Triune will have only missed one renewal, and would still be active gods.
Vivec wont like his people being swallowed by a god he already defeated once.