Probably the big countries like Russia, USA or China because they have the influence and the population to really make a big difference in the market whether it's good or bad, it doesn't matter. We can also benefit if other small countries that are a tax haven can make bitcoin a legal tender because that's more profit for them and for everyone using their country as a tax haven.
You are not reading the news lately no? Trump is against Bitcoin, so pretty sure Biden might not approve of it as well (well the Feds don't like it anyway). China has been banning Bitcoin related activity and have seen a mass exodus of Bitcoin miners to other countries. Russia has mixed feelings about cryptocurrency so this might be possible. Besides Vitalik even though an American might have some Russian contacts as well to promote Ethereum in Russia.
It is very unlikely that major countries such as Russia, the USA and China will follow in El Salvador's footsteps. Because usually big countries are
too selfish to recognize Bitcoin, they still believe in bank system, which of course bank system is more profitable for big countries. So what you
have explained is true, it is very unlikely that a big country will make Bitcoin a legal tender. I believe third world countries will follow in
El Salvador's footsteps, because small countries do need new methods like Bitcoin to help boost the economy. I hope there is one country from
Southeast Asia that will make Bitcoin a legal tender.