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?
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.