Trump seems to have a knack for spotting weakness and opportunities to make money, so Venezuela was the perfect opportunity. Of course drugs were a just pretext and it is all about oil, of which it has a lot.
Maduro had few friends left, was mostly geographically isolated from them (Russia is preoccupied with its own conflict, Cuba is economically struggling too and China is far away) and also had little other support internationally as the legitimacy of his regime was already questioned by many countries.
As to what is next, who knows for sure, but of the four listed only Iran has a lot of oil and is internationally unpopular, also with growing domestic discontent, so that would be my bet.