I want to run a campaign where the main conflict is this group that believes there is one more god in the normal pantheon.  I want this god to be of something ridiculously aggressive, such as being the god of Power and Domination, and I especially want to make him real, but ignored by everyone who worships the current pantheon.  I figure this god will literally be stealing portfolios from other gods, and early on I want to start with Talona, goddess of Disease and Poison, so that his followers can spread a plague to backdrop the game in.  However, the execution of such an idea could be quite difficult, as I can't even come up with a name for this guy.  Any suggestions or ideas are welcome.

How about Chronos?
https://forgottenrealms.fandom.com/wiki/Chronos

Gargauth?
https://forgottenrealms.fandom.com/wiki/Gargauth

There is a one-page adventure that I ran once that dealt with "dead gods," and the one that struck me as the most dangerous was the God of Dissolution.

There Is No God But Dissolution

As her first and final act, the God of Dissolution evaporated into nothingness. This act of divine self-abnegation echoed through the cosmos, an undeniable proof of what had been thought impossible: even the gods can die.

And die they did, in untold numbers--some torn from reality by their kin, some eroded by dalliances with caustic energies. Some merely lost in the trackless expanse of night.

But even in their passing from the world, mortals came forth to worship and cherish them. A brave and mad few dared collect their broken bodies and bury them, or so the legends say.

A tomb of dead gods would be a dangerous place indeed. Fortunately, no such place exists.

If I was a god, I would be terrified of the god whose first and final act was to set the precedent for my mortality. And any new god that might arise and attempt to claim that power would be the fulcrum to trigger a new age of deicide. 

I often find using foreign or ancient languages as a great way to name things. That or obscure or older words in the English language, such as Tregetour - juggler; trickster; deceiver, a person who performs tricks; a conjuror - or Dorobou (Japanese word for thief or robber). Something related to thievery/trickery/swindling seems best to me based off your description -

I'm imagining an underestimated god who grows in power through trickery & cunning until it reaches the height of a terrifying god of Power & Dominion - part of its plan being to spread the lie to its cultist that it is such a god, because belief has power & it convinces its cultists that the world should be a survival of the fittest to encourage them to do nefarious things to find out who are truly fit to survive in this world, while also encouraging them to bring followers to it because it can offer the best of chance of survival in this competitive world. 

Re the plague, firstly beware of magic - there are so many powerful detection spells (detect magic, identify, detect good & evil, speak with dead ...) that can spoil mysteries & also there's a lot of magic that can easily cure diseases (Lay on Hands, Lessor Restoration, etc) - I don't think you should remove the magical solutions to the mystery (I'm not saying you should necessarily take away from the players for being clever to figure out the magic can help solve things, but maybe make them roll insight/investigation/etc as part of the spell - don't make it an auto success). 
You could make a kind magically draining disease - like if you use lay on hands it takes 10 points instead of 5 or costs 2 spell slots - that spreads quickly, putting the population at risk because the magic can't keep up. The questions you'd need to answer would be how did/does this disease spread? Where did it start? Are the individuals responsible in each area that can lead to the organisation? Will non-magical cures be posssible.

That's all I've got for now. Hope this helps. 

Dath Avae'mhaor (Roughly translated from the Elvish as Ending Pain). I used lingojam - common to elvish. There are some good translators to maybe help you find a cool name.

Or Samael...
https://en.wikipedia.org/wiki/Samael

Nos, the God of Nothing, No One, Nobody, and Liminality.

Going in on the God of Nothing is a pretty deep dive, in terms of philosophy and understanding of existential angst, but does fit the profile you not.

No one pays attention to the God of Nothing. So outcasts and undesirables, the dregs of society tend to find solace.

Nobody cares about the God of Nothing, so threats from followers are waved way, dismissed with a laugh.

Nothing can stop them, though.

Because nothing is a liminal Power -- they exist and do not exist, simultaneously. They are part of and not part of, at the same time. They are the moment between being outside and inside, that space between the end and the beginning.

And, let us not forget a truth: Nothing is more powerful than everything.

Give it a name, and make it the god of despotism.

Quote from AEDorsay >>

Because nothing is a liminal Power -- they exist and do not exist, simultaneously. They are part of and not part of, at the same time. They are the moment between being outside and inside, that space between the end and the beginning.

 I would name him Schrödinger...

Quote from Metamongoose >>

There is a one-page adventure that I ran once that dealt with "dead gods," and the one that struck me as the most dangerous was the God of Dissolution.

There Is No God But Dissolution

As her first and final act, the God of Dissolution evaporated into nothingness. This act of divine self-abnegation echoed through the cosmos, an undeniable proof of what had been thought impossible: even the gods can die.

And die they did, in untold numbers--some torn from reality by their kin, some eroded by dalliances with caustic energies. Some merely lost in the trackless expanse of night.

But even in their passing from the world, mortals came forth to worship and cherish them. A brave and mad few dared collect their broken bodies and bury them, or so the legends say.

A tomb of dead gods would be a dangerous place indeed. Fortunately, no such place exists.

 

I was going to run a campaign where, when the characters were about level 20+ (back in 2Ed) they were off of the prime material plane, doing a favor for a demon lord who saved them once when they were lower level. WHile they were gone, the prime material plane was wiped out.  By that, I mean nothing exists. No planets, stars, anything.  There job was to restore it.  To do so, they would have to visit the various elemental planes and convince/compel the various rulers to create planets, stars, etc.  Then visit various other realms to recreate races. I pictured various cosmic things like The Elven Tree of Life in which the leaves that fell became elves, the Dwarven Stone of Creation where chipping off peices created dwarves, and so on.  Other monsters, etc were in a book, whose pages had to be read to create.

As for the Gods, there was a Hall of Dead Gods, that contained statues of all the gods that had ever existed. Some were worn down to near unrecognizability, others were still sharp and clean.  They would have to be restored.  This would give the PCs a chance to become gods.