its not impossible to have a sequel to DII/LOD there are many plausible theories and ideas to consider
Storylines Major/minor -with the worldstone gone demons are more free to enter the world of sanctuary, this is how more powerful demon lords (act bosses of sorts) can come through -as both demons and angels can enter the others realm, I think more demons and angels should be present at the gates to each area, they are suppose to be lock in eternal combat, yet only one angel was in hell possibly fighting demons on their own turf -the last two lesser evil Belial Lord of Lies and Azmodan Lord of Sin elude us still -the eternal battle between the High Heavens and the Burning Hells is still raging -the prime evils returning (or just diablo) without their soulstones as they arent needed for the prime evils to exist -something about Tyrael, is he corrupted? (Kinda been done before, a lot) is he banished from heaven? It may just be me but arent angels forbidden from major influences on mankind or something like that, I forget what it says in the text regards the lore of angels in diablo -Of course if tyrael is corrupted shouldn’t at least one of the seven player chars from d2 also be corrupted, they spent more time in the vicinity of the corrupted worldstone than tyrael did, and im not counting nightmare and hell, unless tyrael is corrupted thru having shards of the worldstone enter his body when he smashed it. -tie up any loose ends from the diablo series not that I can think of any right now but there is probably a few mostly to do with tyrael I believe -the angel inarius, he could perhaps be set free from his prison of mirrors to seek revenge on the prime evils or something -tie in with the diablo novels some how -something with Deckard cain is he still alive, he is getting old, but for some people it would feel like diablo without him. -some mention of the heroes of d2/d2lod, perhaps you could find part of there armor of course the set would have a name like Unknown Barbarian heroes…. or something -you could find out that cain is not the last of the horadrim, there is another, most likely an evil mage or sorts, you kill him then inform cain he once again is the last of the horadrim, cain has a wet eye. -if a playable char returns in d3 like the chars from d1 did in d2, have more of a storyline for them, not just the ‘oh xchar is now a monster I have to kill’ -several powerful demons that have been defeated in the previous games could make a return thru some kind of resurrection/summoning spell, similar to the one Baal used to bring the warlord of blood (bartuc the bloody) back to life -A civil war could break out in Hell, between Belial and Azmodan as they both vie for Ruler of Hell -If D3 is a prequel, you should be able to be involved in the Binding of the three, or the Sin War, or any other event in the diablo History, such as the Blackening of King Leoric (well maybe not that one thats a little recent compared to the Binding of the three and the Sin War) -D3 could be set a short while after the events of LOD and Hell has taken over and you must try to reclaim the World of Sanctuary and possibly Heaven.
some are more plausible than others but hopefully you get my point, there is something t owork with after LOD, i also included a few ideas for a prequel as that is possible too.
about Tyrael...one possible theory of mine is that all three prime evils are actually supposed to be like the four horseman of the apocalypse...except you are missing one...which could be tyrael in disguise...in which case it wouldnt surprise me cuz he destroyed the worldstone knowing full well that the three prime evils could come back...if the 3rd game is based on anything...it might as well be based on bringing the 4th horseman or "brother" to the three prime evils. you can also relate the three prime evils to the 4 horseman...you have war, pestilence, famine, and Death. the only one missing is war. Tyrael is most likely a prime evil in disguise...making every1 think there are only 3 prime evils. it makes sense to me that he would be evil and 4 horseman thing makes sense as well...tell me what you think.
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about Tyrael...one possible theory of mine is that all three prime evils are actually supposed to be like the four horseman of the apocalypse...except you are missing one...which could be tyrael in disguise...in which case it wouldnt surprise me cuz he destroyed the worldstone knowing full well that the three prime evils could come back...if the 3rd game is based on anything...it might as well be based on bringing the 4th horseman or "brother" to the three prime evils. you can also relate the three prime evils to the 4 horseman...you have war, pestilence, famine, and Death. the only one missing is war. Tyrael is most likely a prime evil in disguise...making every1 think there are only 3 prime evils. it makes sense to me that he would be evil and 4 horseman thing makes sense as well...tell me what you think.