I vote for :
One italian CS : I loved the gameplay of Venice in Civ V and I want a civ I can play like that. The challenge to deal with only one city in a game like Civ VI will really be exciting. But it's not necessary that it should be a so commerced-focused civ. An italian CS based on culture would be neat too.
Sweden. This second vote was difficult because I hesitate between a lot. I like the diplomatic and cultural side of Austria, but we have Germany ; I like the silliness of Belgium (a country that defy all common sense and shouldn't, let's admit it, exist), the chocolate country, but to close to Netherlands ; I like Portugal, but we have already a lot of seafaring civ (Spain, England, Netherlands, Indonesia...) ; the Papal State, but I'd prefer an cultured or trade focused Italian civ than the religious one (we already have enough religious civ) ; Ireland (lot of monasteries and ceili) but to crowned with England and Scotland. So, between them, I choose Sweden, because it's neat to have a scandinavian civ that's not a viking one, and Scandinavia is a little empty on the maps.
And, last but most important : Switzerland. As Venice was a totally new gameplay in civ V, Switzerland could be a totally new gameplay in civ VI. Unable tu declare war (but you can defend yourself), emergencies more profitable (history of swiss league against common foe), and the hability to take over more easily foreign cities only with loyalty, even the Capital. Plus, bonuses for gold and a really good defense, and maybe bonuses with mountains/hills, and it would be my all-time favourite civ.