They won't just crown the first placed team champions,whether this happens in La Liga,Premier League or the NBA.
Secondly,it is almost certain that the leagues won't start again any time soon,so there's no way that they'll be able to finish the seasons with the established rules,unless they have games well into summer(which could happen if the Euros/Copa America/Olympics are cancelled),which would be lame and it would fuck up the next season.
So.having some sort of playoffs is the only possible solution.Which again is somewhat unfair,as it radically changes the league's rules.
In my opinion,all leagues should just skip crowning a champion. It's not the end of the world. Still,there would be problems regarding the european qualification and relegation spots.
Anyway,there is also a considerable chance that it won't be possible to play games even next Autumn,so all this discussion is kinda pointless.