There were many lingua francas of which French was supposedly the first global lingua franca. That changed and it became English (from what I understand). We will probably see another language become the lingua franca, so my question is: should it be English? Are there better candidates out there? Why / why not?
We will probably see another language become the lingua franca
That's gonna take a while. Chinese is an unlikely candidate due to how difficult it is to learn to speak and especially read and write, despite the rising international influence of the Chinese state. And I rather doubt that Europe's Germanic-speaking countries will stop using English as a lingua franca anytime soon, it's just too easy to learn for them compared to any other possible candidate.
Let's hope it's not going to be Russian anytime soon.
Something like Esperanto would be a nice alternative for the EU, though. Maybe there's other artificial languages that are even better? I'm not well-versed in this topic.
The Chinese writing system probably isn't so bad with predictive text entry (perhaps even better than English) but I think a tonal language with limited sounds has some challenges taking on the dominance of English in global culture. I like songs in other languages but being able to listen to artists from South America, Mexico, Germany, Italy, Norway, Finland singing in English is a privilege I don't take for granted. When you go down the rabbit hole and discover a country like Japan has a lot of talented musicians it is sometimes hard to understand why they don't have more global recognition until you understand that countries struggles with English language proficiency.
Also English vocab borrows from all around the world. That gives a lot more flexibility for word choice than a language like Esperanto.
French was replaced within 60-80 years. Such change isn't fast, unless forced. However, I don't know how it could be forced. We aren't in a feudalistic system anymore and EU interests are heavily influenced by an English-speaking upper class.
And I rather doubt that Europe's Germanic-speaking countries will stop using English as a lingua franca anytime soon, it's just too easy to learn for them compared to any other possible candidate.
Depends. They have six cases, which is standard for a lot of indogermanic languages, and their declension is mostly consistent. I never learned German as L2, but I imagine the fact that in German cases are not clearly marked on the noun but by the combination of article and noun and that we use two different but very similar marking systems depending on context as utter nightmarish for L2 learners.