You might ask which of the attainable bundles Utopia will actually choose to produce (and consume, since there's no trade yet).  If you want a model that shows that, you can introduce a national "indifference map:"  a set of contour lines representing points that are equally preferred to each other, with lines representing more stuff preferred to lines representing less stuff (Utopians are not ascetics).  You can then combine this indifference map with the production possibility set, and get the one point that the country will actually choose.
This  device is adapted from the consumer's indifference curves that we use in microeconomics.