I am not sure redeeming Leah would really be worth it honestly. Not that she was a terrible character, it's just the whole mortality thing. I think it is OK to let characters die and not come back. Leah served her purpose albeit a diabolic one
The idea of Trag'Oul could work actually. Pretty much anything in sanctuary is mortal, things outside of it like demons and angels do not seem that way. They are constantly recycled or just plain immortal. So the piece of the worldstone was the Eye of Anu if I am not mistaken. It's possible that it's destruction will eventually come around as well. How that would happen is something I haven't thought much about. Though the nephalem souls could be side effect of their mortality and the recreating of the worldstone or something like it.
I had heard (and read) that Skovos isle wasn't intended to be visitable in D3. However that is almost 6 years ago and lots of things have changed with the game as well as probably staff and creative direction too. So even if that was said it doesn't mean Blizzard will or should hold themselves to it.
Skovos Isles to me feels like tristram. It is an iconic place in the world of sanctuary and not integrating it in some way in the future seems like a missed opportunity.
Yeah it could very well be a joke; though I think Inarius and or Lilith are definitely something to contend with in the future. You make a good point about undoing creation though. I think it's all very interesting where it could leave. We have had multiple expansions (LoD, RoS) where Diablo isn't actually the main antagonist.
It would still be interesting idea to go on a limb where if Inarius is the bad guy yet another angel that didn't have the same clout as maltheal had on the angris council. I mean they seemingly gave him up to mephisto without much of a fight. Siding with Lilith or the like to defeat inarius would be a fairly interesting ploy that isn't really out of the realm of possibilities.
I just uploaded a video blog about where the Diablo 3 game can go lore wise after the current Reaper of Souls expansion and patch 2.1.
To conform to forum guidelines here are the main talking points if you do not want to watch the video:
In short it would seem like the Skovos Islands are potentially the next setting for another chapter in Diablo timeline. When playing in Adventure Mode Tyrael and Lorath discuss about a group of Horadrim potentially in trouble who were sent there. I connect that this fairly strong foreshadowing to previous foreshadowing at the end of vanilla diablo 3 when talking to tyrael and how he talks of nothing but malthael.
As far as antagonists go; i talk about using Inarius and or Lilith as the next bad guys. Inarius surely could have escaped his imprisonment with Mephisto and possibly connected with Lilith. Or is even possibly being pursued by her for recapture. There is a lot of ways this could go. However Inarius has a lot of reasons to be another bad guy especially if he is struck with the notion of wanting sanctuary back (since he made after all). I believe Inarius and or Lilith are to be involved with the strong presentation of lore books that are found just before killing maltheal in act V.
I also talk about new classes or at least why i think the most likely new class would be a druid and not an amazon or necromancer. Amazons are a bit too close to demon hunters. It is likely Demon Hunters had inspiration from the Amazon class of D2. Necromancers are touched on since Rathma was offspring of Inarius and Lilith; again Witch Doctors carry the same vibe and I don't think they could be re-introduced without a large amount of backlash. So druid offers some room to introduce new game mechanics and possible roles (healer) much how the Crusader feels very tanky; even though Diablo 3 doesn't need roles like MMOs do.
The video can be viewed on YouTube here http://youtu.be/Ijh3ZIr4I-E it's about 15 minutes long, though the last 5 minutes is just a side anecdote about Inarius and Lilith.
Again these are just my own thoughts and musings. There is no hard evidence or information I have to support anything I say here, so it could be entirely all wrong. I would be curious what others think as well.