First of all, the promo for Medici: The Magnificent, unlike the first instalment Medici: Masters of Florence, is entirely in Italian. There's one teaser I found online with English subtitles, but apart from that it seems like the showrunners went out of their way with this one - the cast seem to be speaking Italian. And it is a big deal.
First of all, Medici: Masters of Florence was a hit in Italy, if not in English-speaking countries, despite the British cast with Game of Thrones start Richard Madden as the lead. The new series is also full of non-Italians - it stars the infamous Sean Bean (yes, let's keep a close eye on him, shall we) and Bradley James - whom I remember as King Arthur from Merlin and who is the least Italian-looking cast so far... We'll see how the blue-eyed blonde Arthur manages to fit in here.
So, is it going to be all in Italian with English dubbing? So far I haven't found any information yet (if you have, feel free to leave a link in the comments, and I'll edit it in here and be sure to spread the word), but that's quite an exciting prospect. That would mean that either all the actors, including the famous ones, had to learn Italian language and learn all their lines in a foreign language - or that we are sure to receive a very well-dubbed Italian version with English subtitles.
Personally, the first option, though complicated, sounds extremely exciting. Although I'd prefer for the showrunners to give Italian actors an opportunity to shine and introduce some authenticity into a show like that, I feel it's a step towards more diversity in film/tv. First you use well-known actors to attract the audiences to the history and the interesting storylines, to make a foreign culture more accessible. The next step is to make shows like that with the proper native casting.
I personally feel that it would be a great step towards both globalization and celebrating various nationalities and cultures - all at once. Can you imagine? Having the success, or the money and resources, of an American/British show, yet to have an opportunity to finally show off your native roots? If the industry doesn't shy away from doing it, a lot might one day come out of this shift.
And it doesn't necessarily mean that all the English speaking actors will be out of their jobs either - they will just free the space for the roles they can't actually fill that well anyway. In the ideal world, that would create a new way of making historical drama - and modern cinema as well.