MxM will be competing directly with LoL, DOTA, HOTS, Smite, and other MOBAs, there have been multiple MOBAs closed down (DC Infinite Crisis, Dawngate, Sins Of A Dark Age, WH40K Dark Nexus)
What does MxM offer that those others don't? Are any of the IPs they own strong enough to carry the game to a large audience (considering the bridges they've burned with their own game communities) The only one of their masters I even recognise is Statesman. it looks like a very generic MOBA from the videos i've seen.
The only interesting mechanic, IMO, is the tag-out (you choose two masters, and can switch between them with a cool down, and the master in reserve regens health/energy). So you can pair a DPS with a tank, or a CC with a DPS, or different types of DPS (sustained auto attack + nuke) switching either as the situation calls for it, or using the tag-out for the brief damage immunity buff.
There's also a lot of PVE content, and PVE-specific challenges. Free players can only do random PVE stages* unless they buy or grind for tickets, which is the only thing keeping PVE from being like handing out a free single-player game. * Although there are some PVE stages that are always available, because they're used for grinding event tokens or parts to unlock masters. The PVE is fairly satisfying, and I enjoy switching from my DPS to Statesman when things get tough. (He's more of a bruiser than a straight tank, too.)
10 of the 30-ish masters are from NCSoft titles, the remaining 2/3 being original characters for MXM, so it's not strictly an "all-stars" game like Smash Bros.
They're going to have an uphill battle, though. Not only because other MOBA are well-established by now and other latecomers have already died, but grinding out in-game currency is hella easy - even if it gets nerfed before going live; and cash shop currency while not strictly "more expensive" is at the higher end of current MOBAs (and can be earned free by grinding). I think the rewards fall well within the majority of gamers' grind tolerance. But if they go and nerf rewards without making the cash shop better bang-for-buck or getting players hooked, they're going to be in for a bad time.