Blizzard got into the act in 2010 by announcing a custom Starcraft II map called Blizzard DOTA. Valve's revelation of its project occasioned some legal wrangling the led to the mod being renamed Blizzard All-Stars. At some point thereafter, it morphed into a full game, which was re-christened HotS toward the end of 2013. Open beta began in May, and live service just over a month ago.
This isn't enough time to get much of a read on what the game's popularity and revenue will be like after they settle in. That said, the overall reaction from both gamers and critics has been quite positive. HotS is more accessible than either LoL or DOTA 2, which suggests it has the potential to reach a broader audience. On the other hand, it's regarded as not having their levels of strategic depth. This may well limit its ability to engage hardcore gamers and e-sports enthusiasts.
For what it's worth, my barely educated best guess at the moment is that HotS will battle with DOTA 2 for the number two slot and eventually win out. I understand that on the whole, this site's readers aren't big fans of designs that are streamlined, simplified, dumbed down or whichever other term you prefer. However, we're not Blizzard's primary target audience. The company is more focused on aiming for a market segment that is far larger, one comprised of players who place more value on ease of access and less on depth. From a business perspective, I can't argue with making games that don't provide the type of play experience I'd like them to.
HoN is a game that brings the same unanswerable question to mind that I ask about Demigod. How much more popular and profitable might it have been if it had come out as F2P? S2 Games' offering made it to market in May of 2010. LoL had entered live service several months earlier, but was far from being the dominant force it has since become. Furthermore, there was no other competition. Market share was there for the taking. I'm not suggesting the race would have been equal or even close, but it surely wouldn't have been as much of a mis-match.
By the time the company switched, which was done in two phases a year apart, the second in mid-2012, it was simply too late to eat into what was by then LoL's two and a half year head start building its player base. Don't take this to mean that I consider HoN to be a failure. What I'm saying is that I think it missed the chance to be more successful. I neither disagree nor concur with them, but there are those who feel it was more faithful to the spirit of DOTA Allstars, which, without the original requirement to purchase, would have helped attract more of its players.
Credible numbers are a fair bit harder to come by for MOBAs other than the tree above, so you should take my conjecture in this area with a sizable grain of salt. HoN's estimated monthly active user count appears to be in the range between 2 and 2.25 million. As for how much revenue the game generates, frankly, your guess is as good as mine. With this caveat, I'll venture that it will be more than $10 million and less than $60 million this year.
It's possible that Hi-Rez Studios' game should be a spot higher on this list. The reason it's not is that although its visibility is higher, albeit this is arguable, the limited data available indicates it has significantly fewer monthly active users. Estimates range from as few as under 700,000 to as many as 2 million. Whatever their number, they seem fervent, witness that they contributed over $2 million to the prize pool for the title's initial world championship, swelling it to $2.6 million. As a bit of context, LoL's for 2014 was about $2.1 million.
Once again, I have no basis for any confidence in estimating how much money this release brings in. That said, since it has only been in service for about 15 months, there's a decent chance its growth curve is still rising at a decent rate. Taking this into account as well as the dearth of potentially huge new entrants any time soon, it wouldn't come as a major surprise if the game rates as the number 4 MOBA a year or even six months from now.
That said, I'm not about to predict a lot more growth. While the game has no crippling weaknesses and offers a few twists on standard conventions, it's not novel or different enough to be widely regarded as a game any MOBA enthusiast must try. As such, it's not going to pull hordes of them away from whichever titles they're currently playing. Indeed, if one or more of the upcoming entries turns out to be a greater force within the category than I anticipate, it's possible that SMITE's upward trajectory could flatten out without rising any higher.