I’ll echo the Foucault suggestion. Almost his entire project was inspired directly by Nietzsche; much of it ends up being an attempt to systematize Nietzsche’s observations on power. It can be tough reading, given your age, but a great choice given where you started. I would jump straight to his mature period and begin with Discipline and Punish and the History of Sexuality, though the best of the essay and interview collections (Power / Knowledge and Essential Foucault) are helpful companions.
Deleuze is also highly Nietzschean, but unlike Foucault he immersed himself in questions of ontology. But you may find that his writing, whilst frequently entertaining and bizarre, is amongst the hardest of all contemporary philosophers. I would suggest getting some help, like Todd May’s lovely little book. Deleuze’s most important work (in my view) is the Anti-Oedipus and Thousand Plateaus sequence; and you may particularly appreciate his Nietzsche and Philosophy.
Kant is still highly relevant, unlike most every other Enlightenment thinker. His moral philosophy and proofs for metaphysics were destroyed (in large measure by Nietzsche!), but the limitations that he places on knowledge in the first Critique remain deeply influential. And it would satisfy the recommendation for a technical selection made by Nihilistic. The best translation of the Critique of Pure Reason is the one put out by Cambridge in their nice blue ‘collected works’ series.
You already mentioned Kierkegaard, which is also a nice choice: aside from Nietzsche he may be the most entertaining philosopher to read. Instead of endlessly dry passages (as in the above recommendation), you’ll end up getting everything from love poems to shopping lists to break the monotony! Good texts are Either/Or, Concluding Unscientific Postscript, and Fear and Trembling. (Or you could cheat and go for Bretall’s decent Anthology.)
I have a great deal of affection for Albert Camus, and in addition to his great fiction work (such as The Stranger and The Plague), his long essay The Myth of Sisyphus should be required reading in contemporary philosophy. The Rebel is also great, and has an homage to Nietzsche. I think Camus is unfairly dismissed these days, on account of the continuing fad for post-(post-)structuralism.
If you enjoyed Nietzsche’s style, you may also dig on Wittgenstein. He is a very pithy thinker, packing incredible wallops into very tight sections. His posthumous master-work, Philosophical Investigations, is often listed as one of [or simply the] most influential books of philosophy in the 20th century. For his other work, save time and get Anthony Kenny’s Wittgenstein Reader.
At some point you’ll need to tackle the Greeks, but given the expose to Nietzsche I would suggest reading well beyond the obvious Plato & Aristotle. Since you’re more interested in reading forward, I would recommend getting at least a good secondary treatment of the period, like Anthony Gottlieb’s The Dream of Reason. And I would not be a very good Pyrrhonist if I did not point out Benson Mates’s excellent The Skeptic Way, which includes a new translation of Sextus Empiricus.
Edit to add: I think the Nietzsche-had-syphilis hypothesis was overturned long ago… 