So: Title of the Blog, meaning that even in this relatively early part of the expansion, I'm pretty focused on getting a lot of characters geared up and making sure they're doing well.
In Legion, more than any expansion before, I've felt compelled to get many characters up to the level cap as quickly as possible. In fact, having dinged 110 on my Druid last night, I only have my Priest left at this point (and I'm eager to level him as I find Shadow a lot more fun now than it used to be - he's done Val'sharah and is level 102.)
One thing I will say though is that after finishing their class campaigns (which is sort of the new level cap - not having a third relic slot in your artifact is a pretty big loss of power, and obviously the class campaigns are pretty exciting to complete) a lot of my characters are sitting somewhat idle. In fact, my Monk just hit 10 artifact knowledge and aside from world bosses, he's basically run one timewalker since finishing his class campaign.
My Paladin, Death Knight, Demon Hunter, and Shaman (the latter being over on the Horde side) have been getting a decent amount of attention, while the other classes (even my Rogue, who has jockeyed for "Horde main" with my Shaman for years) are mostly sitting in their order halls, maybe heading out to do world quests if they run out of resources to pay for artifact knowledge.
There is a big advantage to this approach - once you break an alt out and start playing it again, you'll be able to unlock artifact traits at a really fast rate. While I'm still mostly a Frost Mage stalwart, I'm considering plunking some power into Felo'melorn to see what all the fuss is about - and I just got a nice Fire relic for him.
But there's definitely a lot of remaining motivation to play one's main (or in my case, my top clique of alts.) One big reason is artifact power - while my Paladin is actually only six traits away from getting all 34 regular ones, the fact that there's this constant progression in power really gets you motivated to keep plunking away.
One thing I do wonder about is whether I'll feel motivated to level up same-class alts. My Tauren Paladin, for instance, is mainspecced Ret. But at this point, it would probably be way easier to simply use my rather vast artifact knowledge on my main to fill in the Ashbringer with traits (in fact, I've already gotten two major traits from tossing low-AP "scraps" to this other artifact.
In prior expansions, there tended to be a kind of plateau of power that you hit - if you weren't in some powerful raiding guild going into Mythics or some such thing, you basically got to a point where you had everything you could get out of LFR, or whatever profitable world content there was (like getting those empowered apexis crystal things in Tanaan Jungle) and then you'd sort of be at this point where there wasn't really all that much to do to empower you character.
In Legion, first you've got artifact power, which you can basically always use more of (after unlocking all 34 traits, there's a kind of paragon trait that will very gradually increase your power further and has I think 20 ranks,) but there's also warforged and titanforged gear to consider. And that's not to mention legendary items.
All of this is a good problem to have - one of WoW's biggest issues in the past has been "content lulls," but here there's a lot of motivation to keep working on all of this (though we're still only two and a half months in.) It does mean that some of your alts might be hanging out in their class halls for a while. If your main has more Blood of Sargeras than they know what to do with, you can at least send boxes of order resources to them! And they're account wide, so you can send them cross-realm and cross-faction!