My uninformed guess is that it will contain expansions for existing Essentials builds, and maybe an Essential-ized version of an existing legacy class.
For example, the Red Box has "transmutation" spells for the Mage, like this totally rad daily that turns the target into a toad. It is the best spell I've ever seen for D&D, but it didn't make it into Heroes of the F.
Along those lines, we may see a new Warlock pact, some new Cleric domains (only have Sun and Storm so far), etc.
I wouldn't be surprised to see an Essential-ized Warlord, and maybe some of the other races not yet in 4EE like Goliaths.