I personally do not believe that Legion will have more content than previous expansions, and I feel like the beginning of the xpac is very telling of the rest of it.
World quests are just dailies with some better rewards (for now, the rewards will become eh soon), heroic dungeons now have a weekly lockout (mythic = heroic, mythic + = mythic this expansion), drawn out garrison class quests (not that bad, but it's a major time gate, would have preferred "clear all dungeons" or something), and they've shown they are planning on releasing raids more spread out, but it's clear they aren't planning on doing "more" raiding content, just more spread out. It's like taking HFC, splitting the bosses into 3 groups, then releasing 1/3rd at a time every 2 months.
It's clever to keep people playing the game, but without MORE content, it will end up feeling exactly the same, perhaps even worse because we have less bosses to clear weekly.
Why can't Blizzard just make MORE content, rather than resorting to extreme levels of time gating to keep people subbed?