I honestly loved her ability in Civ V
I too! I once played with her in giant earth on civ5, I remember to start in Rio Grande do Sul (in South America) and spread my cities all over the globe, because her marriage alliance ability make it very easy to grab other cities. But I think it is a bit over power ability because is too easy to grab cities with marriage, you just to need to wait 10 turns after become suzerain, and in civ 5 is possible to become suzerain with money, so I just invest in money and purchase cities all around the globe, and not small cities as settles one, no, it was great city states with infantary and etc.
Maria Theresa as a leader (either Austria, or a possible dual one for Germany/Hungary)
Since civ6 don't will add more civilizations, it's impossible to she become leader of Austria.
Should be fun have Maria Theresa as leader of Hungary, despite I think the civ of Hungary is made about other historical period to fit better in Hungarian narrative of history. And German should be very weird if she leads off, mostly because she lived in a time before Germany, of course there was already a concept of Germany, and a willigness to do a Germany country, but Maria Theresa's Austria was out of Prussian's plans of Germany in the end, so she never leads Germany.
But, if Ana Nzinga is leading Kongo... I don't will be surprise if Maria Theresa appears leading Germany....