It probably wouldn't be worth Mega Bloks obtaining the Back to the Future license. There's no current media for it, which means there wouldn't be much interest, ie sales. Sure, [competitor] made the Deloreon. But it's only one set, and the [competitor] brand name really helps sell it.
Ghostbusters does have a new movie coming out. Maybe they license it, but it's not really a proven property for merchandise in today's market. Also, there might not be enough material there to support a line of product.
Masters of the Universe does seem like the most logical property to fill the fantasy genre for MB. Mattel owns the property, so there are no licensing fees to pay off.
I doubt MB ever licenses Aliens or Predators. They both tend to be rated-R properties, which toy companies tend to shy away from. Aliens and Halo have such similar design aesthetics it might not be worth buying up another license only to make similar looking material.