Well evidently the butcher that we killed in D1 and D3 are actually different butchers. Despite the name: “The Butcher”, there’s actually many of them according to the lore book you get when you kill the butcher in D3:
“The truth about the Butcher demon was more startling than I ever anticipated. Evidently, there are many of them within the Burning Hells, though they used to be solely under Diablo’s command. The butchers are stitched together from the parts of other demons to combine their strengths, then given life through wicked magic.”
As for the Countess, well apparently she had died even before the events of D2, as she was buried alive and the tower that we fought her at served as her tomb, and going by what I’ve heard thus far in D:I, yes she can apparently return also. Leoric’s mind was pretty much broken due to Diablo’s influence, and as such the corruption had left him a vengeful ghost, so yeah, he can technically return as well.
Honestly it probably won’t. The only real way of ending the Eternal Conflict, would be if either the High Heavens or the Burning Hell (or both) were actually truly destroyed or purified out of existence (in the case of the Burning Hells) or corrupted beyond redemption (in the case of the High Heavens). We saw that the core of the High Heavens is the Crystal Arch, if that’s either destroyed and/or corrupted, then the High Heavens will fall alongside it. The Burning Hells likely have something similar, imo it might be the entirety of the Burning Hells.