Who would verse who? Murika v Chi-nah? More importantly is who Mother Russia will back.
Take your pick really. Nationalism is a broad brush, there are different forms of nationalism. If Nationalism does increase in the near future as a result of Covid 19, the chance of a massive war goes up substantially. The world was kind of heading that way anyway, almost everyone who can remember WW2 has gone, and the World trade and Globalisation that was begun in the late 40s and 50s specifically to stop another world war from ever happening again were starting to come under threat. Covid 19 has massively insanely accelerated the whole process.
So who might be the key players in a massive war? Here are some speculative views:
I'd pick China to go back into it's shell. China IS very nationalistic already, which is part of the reason it has done really well in the last 30 years, it has been operating in a context of unprecedented levels of Globalisation. China historically has been very insular, and as it sees nations around the world inevitably becoming more nationalistic and self-reliant it will probably go back into hiding. The fact that Covid 19 originated in China is also going to be a factor, the third world countries in which China has been trying to gain influence are probably going to cop it badly because of Covid 19, and do so for years. They won't be nearly so happy to see Chinese diplomats come knocking any more. China has tried to mend the situation by sending aid, but most of the test kits they sent don't work properly.
The US was trending towards Nationalism, but was still more or less fulfilling it's role as the world's policeman in the context of Globalisation. I suspect that the way things will go will mean that the US will turn towards isolationism regardless of who wins the Presidency in the coming elections. Not because of ideology, but because they won't have much option. The US is already getting hit hard, within about 2 weeks their death toll is going to go past Italy, and its going to get a lot worse before it gets better. Sooner or later they are going to start putting red lines through their overseas expenditures, whether that be military expenses or the billions they spend projecting soft power. They will risk substantial civil unrest if they don't pay significant attention to their domestic situation.
Europe is where the previous World wars kicked off. Keep an eye on threats to the EU, the current crisis is going to place a lot of strain it. The EU does two main things; it opens up borders, and it combines economies. Covid 19 has closed the borders, and the unequal manner in which member economies are going to be hit will make a combined economy harder and harder to maintain.
In all this Russia is perhaps well set. Covid 19 will effect their built up areas, Moscow already is taking major action against it, but the general populace of Russia is genuinely tough, they have a different mindset compared to the general populace of the US, UK, and western Europe. Russia is also typically positive for net exports, which means they have a good degree of self-reliance, they are generally relied upon. Russia has excellent access to natural resources, which puts them in good stead for an increasingly nationalistic international environment, and they tend to do well in both manufacture and agriculture. This is really important. Take for example Japan in WW2. Japan relies heavily on imports of raw material for its manufacturing. Australia stopped selling them iron ore in 1938. USA stopped selling them iron ore in October 1940. For Japan to maintain it's nationalistic ambition it had to try and take iron ore by force, which of course brought the US into the war after they bombed Pearl Harbour. Because Japan didn't gain significant sources of iron ore it couldn't produce weaponry on the scale the US did, and ultimately it lost the war. Russia doesn't have that problem. If it can ride out the Covid 19 crisis it could be well positioned. And I reckon Mother Russia might be holding a few grudges over the actions of NATO and it's neighbours over the last few decades. Look up "three seas initiative" for an example of something Russia is not happy about at all...
But what might actually trigger a major war? Hard to know, but if I were Israel and Palestine I'd be having a good hard think about patching things up and making out like they are friends pretty soon. Most sources of potential conflict are relatively localised, but that is not the case in the middle east. If Russia wanted a pretext to project hard power into the Mediterranean and Gulf regions, it is going to find the Middle East a pretty tempting target. If the US backs down as global policeman, Russia might decide it is time to sort out the Israel-Palestine issue "for the good of an already suffering world".