Now I have two undefined terms, rather than one.
I'm not trying to be sophist here, I'm just pointing out that "classical utilitarians" are following a complicated, mostly unspecified utility function. This is ok! There is nothing wrong with it.
But there's also nothing wrong with having a different, complicated utility function, that captures more of your values. Classical utilitarians do not have some special utility, selected on some abstract simplicity criteria; they're in there with the rest of us (as long as we are utilitarians of some type).
Most people's ethics are based on their desires. People's desires are based on what makes them happy. That's as far down as it goes.
A somewhat simplistic definition of happiness is positive reinforcement. If you alter your preferences towards what's happening now, you're happy. If you alter them away, you're sad.
Some people see never-existed people as moral agents, and claim that we can talk about their preferences. Generally this means their personal preference in existing versus non-existing. Formulations such "it is better for someone to have existed than not" reflect this way of thinking.
But if the preferences of never-existed are relevant, then their non-personal perferences are also relevant. Do they perfer a blue world or a pink one? Would they want us to change our political systems? Would they want us to not bring into existence some never-existent people they don't like?
It seems that those who are advocating bringing never-existent people into being in order to satisfy those people's preferences should be focusing their attention on their non-personal preferences instead. After all, we can only bring into being so many trillions of trillions of trillions; but there is no theoretical limit to the number of never-existent people whose non-personal preferences we can satisfy. Just get some reasonable measure across the preferences of never-existent people, and see if there's anything that sticks out from the mass.