In an interview today, Ion Hazzikostas (WoW's new game director - though he's been a pretty high-profile guy on the team for years now) talked about the future of the Legion expansion, and it looks like we've got a fairly clear path for what we'll be getting as the expansion unfolds.
7.1 is obviously already out, with Trial of Valor filling out a kind of "tier 18 and a half" with Emerald Nightmare, and with Nighthold coming out as the true tier 19 raid (meaning that by the end of the first official raid tier, Legion will have 20 raid encounters.)
Nighthold isn't going to need its own patch - theoretically it was considered a 7.0 raid, but that's a little absurd as we've clearly passed 7.0 and have not yet stepped inside. But the point is that at the end of the extended Suramar City campaign quests, the raid itself will open up, and that's going to happen some time in January.
I suspect that they're going to take their time releasing 7.2. They're letting Emerald Nightmare and Trial of Valor breathe for a bit, and with Nighthold comprising just as many bosses in a single raid, I imagine that we'll have that as the main content until the middle of 2017.
Blizzard will, I'm sure, be looking closely at how people are doing, but I for one would far rather wait a month or two longer for 7.2 than have another 14-month stretch of zilch at the end of the expansion.
With no clear release date, but definitely a significant amount of work already done on it, 7.2: The Tomb of Sargeras will put a lot of content on the Broken Shore zone (which is currently accessible but beyond the Kosumoth quests, doesn't have a lot of stuff for you to do.)
The patch will add a new faction - a kind of united front between all the Class Orders - and will contain a new class campaign that rewards flying class mounts for each class.
The headliner will be the Tomb of Sargeras, which will be the official Tier 20 raid. This raid will have Kil'jaeden as its final boss, and also contain a fight against the Avatar of Sargeras that was buried here way back when. The raid will have 9 bosses, taking the expansion's raid boss count (not counting world bosses) to 29, meaning that it will have already beaten Cataclysm.
7.2 will also add a new dungeon called the Cathedral of Eternal Night, which is linked to the Tomb of Sargeras raid (similar to how the Frozen Halls dungeons were linked to Icecrown Citadel.) This will have four bosses and have you climb to the top of the ancient temple, while the raid has you descend into its depths. We don't have any details on the fights or characters within, but we do know that there will be normal (I think normal,) heroic, and mythic/mythic+ modes for this dungeon.
7.2 (unless they do this earlier) will also bring about heroic versions of the Arcway, Court of Stars, and will create two heroic wings of the Return to Karazhan dungeon that can be queued for separately.
We're also going to be seeing some tuning done with Heroic and regular Mythic dungeons, raising the difficulty and rewards, while level-cap normal dungeons will probably be rewarding higher-quality gear (I believe the current plan is 825, which is what heroics currently reward.) Expect numbers to shift a lot while this happens, but the presumed goal is to make sure that these remain relevant content and provide a more effective stepping-stone for characters who are fresh to 110.
7.3 will have us journeying to Argus. The planet will be represented by a single zone, and will presumably have a lot of outdoor content to play through. Argus will represent the capstone on the Legion experience, and will thus presumably contain its final raid and, hopefully, new dungeons as well.
That said, given that we are several months away from even 7.2, I don't expect we're going to see much of 7.3 for a long while.
Still, the content that they have announced looks highly satisfying. I'm certainly not eager to set aside Legion anytime soon for whatever comes next, and I'm glad to see that Blizzard has a lot of stuff planned.
There hasn't been any word on a "small patch" like 7.1 coming in there, but to be fair, for all we know the 7.3 patch will be the first half of a two-parter, perhaps with 7.4 actually introducing the final raid. We know very little about the final patch other than its location, except that our purpose there is to have a much more definitive victory against the Legion than simply closing the portal at the Tomb of Sargeras.