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.