There are a few that come to mind. There's the perennial examples of Jerusalem, Hong Kong, Macau, and Istanbul -- but these seem less likely than ever. Istanbul is totally a part of Turkey; meanwhile, China's control over Hong Kong and Macau, and Israel's over Jerusalem, are more secure than ever, and that doesn't look like it's going to change.
However, I can see Aden potentially seceeding from Yemen. It has changed hands several times in the Yemeni Civil War, but it remains the de facto capital of the Southern Transitional Council, the UAE-backed government. That's one of four main factions in the Yemeni Civil War (the other three being the Houthis, based in/around North Yemen; the Saudi-backed Presidential Leadership Council, which controls most of Hadhramaut; and various al-Qaeda and ISIS-affiliated factions, which don't control a lot of territory but sort of proliferate throughout the country).
It's possible (though probably not too likely) that Yemen will split up into North Yemen, Hadhramaut, and Aden. If I had to put money on it, though, I'd say it's more likely that Yemen splits into a GCC-backed South and an Iran-backed North; or more likely still, that it doesn't split up at all, and remains a theatre for confused endless fighting, like Syria or Iraq.
Another possibility might be Sevastopol. Russia already considers Sevastopol a federal city (alongside Moscow and Saint Petersburg), meaning at least officially, it's administered autonomously from the rest of Crimea. Of course, Sevastopol is under Russian military occupation just like the rest of Crimea, so it's hard to see this as anything more than a symbolic status right now. But who knows how the Ukraine War will turn out? I don't think Russia's occupation of any Ukrainian territory is sustainable in the long run, but I guess all things are possible. And maybe Ukraine and Russia will compromise and create an independent Sevastopol. It'll be just like the Free City of Danzig!
Finally, there's Nigeria. On one hand, Nigeria is one of the fastest-growing economies in the world, and is making great strides in development. But on the other hand, there are major divides between North and South, urban and rural populations, and between the rich and poor. There are several secessionist movements, from the predominantly-Igbo region of Biafra, to the Hausa/Fulani Muslims in the North, to the Yoruba people in the southwest -- and these groups (and others) are unevenly represented in the military, government, economy, etc. What's more, despite being a republic, several Nigerian states have monarchies. For example, the Muslim states of Sokoto and Kano are ruled by sultans as constitutional monarchies. I wonder if Sokoto would ever proclaim independence from Nigeria? I doubt it -- Muslims make up the majority of the Nigerian military, and Sokoto is on the front lines against Boko Haram, so succession isn't really in anyone's interest at the moment. Ditto for the Yoruba Christian-majority state of Oyo. But, if Nigeria collapses into civil war again, maybe all these little kingdoms -- united into a federation, like the UAE or Malaysia -- could break away?