The brain structure of marsupials is very primitive and as a result they are relatively unintelligent, brain size compared to body-size is a decent indicator of intelligence, but it's not the only variable, brain structure is important too. I would honestly say that marsupials would need a long time to become as advanced in brain structure as primates.
Excuse me? Don't accuse me of that, thank you very much. It is common knowledge that marsupial brains are structured differently than those of placentals, partly because they diverged earlier, but this difference in structure has a lot to do with why they are less intelligent than placentals. I am NOT perpetuating a myth, this particular fact is true. Marsupials are better at lots of things than placentals, the Thylacoleo was the most specialized predatory mammal arguably ever (the teeth), kangaroos are more energy-efficient over a long distance in endurance and simply that bounding is better than galloping, kangaroos compete successfully with wild goats, etc etc etc. It doesn't mean I'm perpetuating a myth if I'm pointing out a fact of anatomy, ask someone who actually knows about marsupial anatomy and they'll tell you.
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).