What arguments exist against inequality in life fulfillment?
By life fulfillment I mean the things that one is able to enjoy in one's life and why not all people can enjoy the same things (because of e.g. division of labour, wealth inequality, inequal division of natural resources etc.).
Is life fulfillment only a matter of the individual or does it concern communities or societies as well?
What practical advances exist for improving the situation of all humans in the "pursuit of happiness"?
Take a look at Consequentialism, or more specifically at Utilitarianism. One branch of Consequentialism is act Consequentialism (Utilitarianism simply being hedonistic act Consequentialism), which is briefly summarized as follows by SEP. "Act consequentialism is the claim that an act is morally right if and only if that act maximizes the good, that is, if and only if the total amount of good for all minus the total amount of bad for all is greater than this net amount for any incompatible act available to the agent on that occasion." (https://plato.stanford.edu/entries/consequentialism/#ClaUti) Basically consequentialism attempts to maximize utility, in other words general well-being, of all people. I think this is more or less what you're looking for.
What arguments exist against inequality in life fulfillment?, Is life fulfillment only a matter of the individual or does it concern communities or societies as well?
Humans are social animals, we have evolved to exploit niches which are best exploited through co-operation. For this reason we have structures in our brains which drive us to form social groups and which recognise (through re-enforcement neurotransmitters) that the welfare of that social group will also affect our own ability to survive. Thus whilst an entirely selfish person could well thrive in today's society, the feedback mechanisms in their brain which provide feelings like 'happiness' or which feel psychological pain would respond negatively to the lack of welfare within what that person considered their social group.
This is not to say that the positive feedback from having lots of other desires satiated by wealth would not function to make that person happy, but that there would exist a greater state of happiness where those desires are satiated in addition to the desire for a thriving social group to provide opportunities for co-operation.
What practical advances exist for improving the situation of all humans in the "pursuit of happiness"?
For the reasons given above, virtually all ethical philosophies consider maximising human happiness as their objective. As monster319 points out, the most recognised of these is Utilitarianism, but the objective is true of all moral philosophies, it's only the means which changes. Kant does not propose his use of categorical imperative at random, he does so because he believes following such well-derived maxims will lead to greater overall human happiness. Religions do not randomly prescribe actions, they do so because they think that following them will lead to a happier society, either now, or in the afterlife. Again it is only the means which changes, not the goal. So with regards to your last question, virtually all moral philosophies consider themselves "improving the situation of all humans in the "pursuit of happiness", even if some do not openly admit the fact.
The key part of your last question is the term 'practical'. This implies methods which actually work. Questions about the results of practical applications of certain moral philosophies can only be answered by scientific investigation, and so your studies in this regard are better directed to sociologists, psychologists and cognitive scientists. There are SE sites which cover these topics and you might want to consider posting the last part of your question there.
