Iran-Israel (not entirely sure how that’ll work since they’re not directly adjacent)
North Korea-South Korea
I see the world coalescing into two major axes of power: “the West” and Western aligned/allied nations comprising US/UK/EU/Australia/Israel/Japan/S. Korea on one side. Largely Democracies or Parliamentary systems.
On the other: Russia, China, North Korea, Iran. Largely Autocracies, and one Autocracy with Chinese Characteristics™. All four have beef to pick with Western countries. They might press their advantages if they see that Western support for each other wavers (Ukraine and Israel). We’re already seeing them warm in relations with each other (Putin/Kim visit, arms deals between Russia/China and Russia/N. Korea). They might form their own NATO style Bloc which would be stronger than the paper tiger that is the CIS. Combined, they may very well have the ability to split the West’s priorities if internal division is strong.
I think it’s going to get even more tense the further this decade goes on. The dice that is the US Presidential election may decide the next big moves of these nations.