Blizzard's barking at the wrong tree with this one. The MOBA genre is popular among gamers who are willing to put serious time in it, not casuals, and I doubt many gamers will go for a dumbed down MOBA just because it features Blizzard characters.
All forms of games have more casual versions, and in the main they are more popular because they are more accessible to casual players. Perhaps the reason why MOBAS only tend to be played by 'hardcore' gamers is the very fact that current titles are designed in a way where you have to invest so much time into them.
A more accesible and casual MOBA? I think you may be surprised how many people would play it, and in fact it may do the MOBA scene a favour. It gets people into the genre and can be used as a stepping stone into more advanced titles, which can only be a good thing?
But I agree, the wider issue of dumbing down games to suit casual players is frustrating. Just look at what they have done to Quake Live!