Yes, maybe a bit like Heroes of Might and Magic, turmoil between the 'good' factions.

I can see the Titans re-originating the planet as potentially greater threat, or Old Gods breaking free and corrupting shit without collapsing the ground.

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Ehh the story can't really "END" without all evil being destroyed and it's pretty much impossible to kill all evil in this game.

Because the original demons (Nathrezim) don't really die much (they just go back to the Twisting Nether for the most part)
and Old Gods don't die without destroying a planet in the process.

So evil will always be around.

It just might not be as organized as it is right now.

I'd say evil could in theory be destroyed in a way. Remove all the "black" from the world but leave enough "dark gray".

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Can the Legion be completely destroyed? Not too sure to be honest.

A war against the Legion or the "DARKNESS" is going to be one where where a whole bunch of cosmic powers are going to be clashing. Things are going to get pretty gnarly pretty quick and i have a feeling that there's going to be a little more with the struggle than GOOD vs EVEIL.

The Burning Legion as an Orginisation run by Sargeras can be destroyed. Demons and such will still be around and do whatever they did before Sargeras banded them together for his cause and whatever those who don't belong to the legion are doing right now. but by defeating sargeras the orginisation known as the burning legion the greatest threat to existence we know so far will be gone.

there willstill be danger: feuds between races or some of the surviving demons under sargeras might still want to do what he started. war will never be gone.

example: even with the death of the imperator there is still war in star wars as there was before him.