Mills and Kant wrote their classic treatises quite a while ago. There are people who have modified and adapted their theories and applied them to more modern times. John Rawls is perhaps the best known and most accessible. This Rawls obituary gives a very readable overview of his theories.
A quote from that obit:
If there is a single principle at the centre of his system, it is that basic civil and political rights are inviolable. Rawls believed, following Kant, that from the moral point of view, the most distinctive feature of human nature is our ability freely to choose our own ends. It follows, on his account, that the state's first duty with its citizens is to respect this capacity for autonomy - to let them live life according to their own lights, and to treat them, in Kant's phrase, "never merely as a means, but always at the same time as an end".A leading feature of Rawls's theory, then, is the the priority it gives to the right over the good - to claims based on the rights of individuals, over claims based on the good that would result to them, or to others, from violating those rights. Put another way, he argued, in opposition to utilitarian, perfectionist and communitarian principles, that the first duty of the liberal state was to safeguard the individual's basic civil liberties, and that "the loss of freedom for some" can never be "made right by a greater good shared by others".
Thanks for being willing to engage in the discussion!