As I've said before, we shall not forget about the Thieves Guild.
"This isn't over, the rest of the Thieves Guild will find you..." - Nigel Cutthroat
Let's say there will be two expansions.
In the first one:
Diablo is weak, hell is weak, all demons are weak, right? We saw to that in the ending of D3.
Now adria is still alive in thoughts on how to rescue poor Diablo and his minions for a great sucsess.
Since she got no prime evil to lean back to, she will gear up with humans, the thieves guild.
We acually gets to fight humans. (now would be the time to add cool cities and towns, a big forest to fight in..slightly diffrent from the other palces we've been visiting earlier.)
In her search for the soulstone we have to stop her, right?
Now what would the angels think of this? chaos between humans?
We all know, more or less, that the angels voted once for the human race to die..
Now there could be three sorts of endings on this expansion..
1. imperius, the angel see no other option then joining up with diablo, become one and end this (since the angels are also weak from the attack). Diablo restore his power and all hell brake lose!
2. Adria restore diablos power on her own and we have to fight all three sides, humans, angels, demons..our only alliance would be tyrael, leah, and perhaps a the blacksmith and jeweler. (perhaps the templars aswell)
3. We speak sense to the angels and restore peace with them, but adria still got what she wanted, diablo. (After that, we fight adria, kill her, but fail to stop the "healing" on diablo. Some sort of the ending boss for the 1 expansion)
Now all those have a small twist, right?
The second exapnsion:
If 1 ending: Diablo have won, the whole world is in ruins and there is no hope left, or is it?
We are on our own, picks up the fight against the angdoms (a word for angels and demons)
If 2 ending: Pretty much the same as the one above.
If 3 ending: This is more or less likely to be same old diablo 3 concept. (Diablo 3 story all over again, execept for a few diffrent things like, locations, characters and so on, but the main goal would still be the same)
I don't know, I just came up with something..anyway..I would really like to battle a few humans :)