Can WoW's decline be slowed or maybe even arrested?
I think there's a bigger question here, which is how much Blizzard actually wants these things to happen. It's tempting to say that with the amount of money hanging in the balance, there's simply no way the studio won't fight for every possible dollar. Frankly, I see this as at least somewhat over-stated. From a business point of view, the issue involves more than just WoW. It's a matter of gauging how to allocate resources to obtain the best possible return. So, the execs have to decide how much support the game should receive as opposed to using the same money, people et al on other projects.
My guess is that the brevity of the spike due to WoD won't increase their willingness to spend on WoW. Indeed, I suspect it might make them less eager. Yes, the expansion sold a few million copies. Looking just at the number itself, that's quite a few. However, let's remember that the potential target audience is the total number of accounts created, which is in excess of 100 million. Seen from this perspective, the percentage uptake / penetration rate is in the low to middle single digits. It may well be far more significant is the fact that overall, people didn't stick around, which means they didn't generate much in the way of ongoing revenue.
Your opinion may differ, but Legion doesn't look to me like it will bring anything that's likely to produce greater retention. At best, I'd anticipate it providing a brief spike like WoD did, but as discussed above, it will probably be building upon a considerably smaller base. I can only speak definitively for myself, but I strongly suspect that a fair proportion of this site's readers are, like me, less likely to return with every new expansion – if you haven't quit for good already.
What are the chances of a sequel or at least another Blizzard MMORPG?
So as to be clear, I'll state up front that I'd very much like to see the studio make another MMORPG. Whether it would be a sequel or something completely new doesn't matter nearly as much. Indeed, I've wavered on this matter; for what it's worth, I'm currently leaning slightly toward the latter. That being said, my gut tells me neither is particularly probable. So, I'm not holding out much hope of one being announced any time soon. And even if one is, we'd still be years away from being able to play it.
I could easily argue that Blizzard should be looking to continue drawing revenue from players who leave WoW. Actually I'd be completely shocked if this thought hasn't come up. However, shifting them into another MMORPG isn't the only way to achieve this goal. It would be more cost-efficient, possibly much more so, if they can be moved into some type of game that involves lower development and/or operating expenses.
In addition, if Blizzard were to make a second MMORPG, it would face a very high bar in terms of expectations, probably both internally and externally. The former is far more difficult to assess, but if you were the decision-maker, how willing would you be to greenlight a project that isn't absolutely enormous, with a correspondingly huge development budget. Sure, it would very likely make a profit, but that would come years down the line, and until then, you'd have to absorb hundreds of millions in expenses. Compared to that, the prospect of multiple smaller (although sill large) endeavors that start bringing in money much sooner has to look appealing.
Will WoW go free to play?
For some time now, I've been of the opinion that it will, and I haven't seen anything lately to alter my thinking. So basically, I still think it will happen first in those regions where F2P is most deeply entrenched, with China being my pick to be first. I realize WoW has been very successful there as a P2P release, but as the size of the player base there continues to drop, there will be less reason not to shift over to the business model employed by the large majority of titles, a few of which have even larger audiences. In addition, changing over won't be a big deal for the regional publisher, Tencent, or for its customers since the company already operates a sizable portfolio of F2P offerings.
In North America and western Europe, the proverbial bird in hand is still very large. If we estimate 2.5 million accounts at $15 per month, it works out to $450 million per year in gross revenue. However, this is an annualized total that, as discussed earlier, is shrinking, quite possibly at a fairly quick rate. So, in a year to two, there will be less for Blizzard to lose and more to gain. I doubt we'll see WoW go F2P here for a while yet; the earliest I can imagine is a few moths after Legion launches. However, I do believe we're continuing to inch closer.