I don't think Sylvanas will be warchief for the whole duration of Legion. Either Vol'jin will come back or someone else will become warchief during a major patch. They should have a voting event like Azeroth's chopper for warchief.
Blizzard was seriously considering Lo'themar during SoO, he might end up being the warchief. Saurfang is not physically in his prime true but being a warchief is not about being physically the strongest. He is a tough war veteran but he has also seen what war can do to people. He would be the best warchief for the horde.
People used to admire Thrall as warchief before Cata. Then Blizzard ruined his character world shaman, Goel, married to that annoying Aggra. If Blizzard decides to make him warchief again then his character needs to do a 180 degree. Get Aggra killed in Legion and make it some how Alliance's fault (as in they could have stopped but didn't intervene). Make him a bit jaded, cynical. Right now he is too goody two shoes to be the warchief of the Horde.
To be honest i'd prefer Undercity, really wasn't a fan of the new Orgrimmar layout, much preferred the old one.
Why is it going to be weird, again? You're still going to be hanging around Orgrimmar or Dalaran.
Currently, Genn is certainly more "present" on the Broken Isles, but Anduin is still King.
As for the "official" capital, players will go to where things are most convenient. UC has facilities closer to each other, but Org is more open for flight and has easier access to zepplins.
Panic if Silvermoon became the new capital and it still isn't updated =(.
All the portals that are in Shrine (mop) also added to every city in game. People idle where they want then. Especially void storage and transmogrification to every city.
I smell settling for Northrend, Eastern Kingdoms & Udnead Campaign- > Forsaken, Gilneas, Lich King, Dwarves, Alterac, Kul Tiras, Karazhan, Quel'thalas etc.
Ohh no worries there--he'll still die like normal, the description just says hes pretending so its funny.
And off-topic, but speaking of killing the Prophet Velen, I heard the funniest story a really long time ago that a horde raid went into kill him once on my realm (Azjol-Nerub) sometime during Wrath, and the whole raid got lost so bad that they couldn't find him and just hearthed out lol
edited because after I thought more about what you wrote, I see what you mean--but ah wells