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.
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.