Sorry, no direct buses from DTD to ANY of the theme parks. You'll have to get to a resort first, then bus from there.
However, in the case of Epcot and MGM, you can bus it to one of the Boardwalk area resorts and then walk to Epcot or MGM. Or take a boat from the resort to one of those two parks.
I don't have my Epcot/MGM/nearby resorts geography down pat, but the general idea is correct.