There is one country that, on paper, would be a prime candidate for nuclear weapons. It’s large, militarized, has had a reputation for what can politely be called “stubborn independence”, and directly bordered the USSR. The country in question: Turkey. Now, there has been constant talk and pushing for a nuclear arsenal from it as early as the 1960s. But it has not amounted to anything substantive in actual history.
That could very well have not been the case, and archrival Greece might have followed with an (attempted?) independent deterrent of its own. From there, the butterflies could spiral off. As someone who is no expert on the politics of that region, I will make no claims. But as an avid reader of cheap thrillers, I can safely say that in that situation, Turkey and maybe Greece would join the USSR and Pakistan as the countries of choice where the terrorists buy/steal/are donated nuclear weapons from in novels and their adaptations.
Actually I’m a little surprised that there’s been fairly little use of South Africa as a nuclear source given the apartheid government’s easy villain use and its genuinely successful weapons program. I guess the South African nuclear arsenal was too small (it amounted to only six Little Boy-level warheads) and more importantly, too obscure (it didn’t stay in the headlines long because the ANC government rapidly dismantled it with very little controversy).
Of course, if postwar Japan with its technology and piles of fissile material managed to go nuclear (some fire-breather rises to the top of the ruling party?), you can bet what a bunch of 1990s technothrillers would have focused on.