Summary: The Doctor hopes to someday bring peace and harmony to the whole universe.
Universal peace and harmony, with every race in accord, is perhaps unlikely but the Doctor sees no harm in hoping it will eventually come to pass. Admittedly some races aren’t genetically wired for peace, Daleks, Cybermen, and humans among them, but it would hardly be any kind of challenge if they were.
The universe is such a vast, varied place, filled with so many sentient races, some peaceful and civilised, and others not. Bringing them all into harmony is a task that could take all of eternity, but it’s a goal worth striving towards.