Well, no, not really. The baby marsupial is born as an embryo, essentially, it would need longer pouch care, not gestation. Also, marsupials are far more primitive mammals, that's far more likely to be the reason.
Thanks for asking, but there are already lots of primate-marsupial portrayals, the Neocene has the marsupial lemur, and Dougal Dixon's "After Man" had the marsupial monkey, anything evolved from an opossum would be basically identical to such things. If you have any other idea you want me to illustrate, tell me.
A longer gestation will allow the organism to be born less embryonic and thus a faster metabolism to feed brain-power. If this keeps up as an evolutionary trend, certain Marsupials will cease to be Marsupials.
Just imagine if Opossums evolved into more-humanoid forms in the far future! We'd get a Metatherian humanoid.
But it won't gaurantee it. Kangaroos have a more advanced brain than other marsupials, and they are still marsupials, they might have a higher metabolism though.
I'm sorry, I think a humanoid marsupial is a terrible idea, anyhow, I already drew one for The Speculative Dinosaur Project, except it doesnt have a large brain. The only way that humanoid beings will appear again is if they are either descended from apes, or humans. A marsupial primate would most likely look vaguely like a koala, the size of a gorilla, with 2 thumbs (which koalas have btw).