The sales of the Fiat 500 in the US do not justify a facility to build that model here. The 500 is built on a different platform than the Smart based Grande Panda. Is there a business case for building vehicles based on the Smart platform here? I think so, but I don't know which facilities are available for that.
Fiat has three new Smart based vehicles in the pipeline, a C-segment SUV (F2U), a fastback C segment crossover (F2X), and a global lifestyle pickup similar to the Strada currently made in Brazil. (The term "lifestyle Pickup" is an American idiom that I use, Europeans don't use that term.) The next Jeep Renegade could be derived from the F2U and the fastback crossover could be a Chrysler or Dodge model. If the vehicles are built here, they could have styling unique to the American brands. The Grande Panda could also be built here avoiding any tariffs.
Some of the European enthusiast websites are saying there is a very credible rumor that the 1.2 liter, EB2 Gen3 three cylinder engine will receive all the same upgrades that Dundee, MI built 1.6 liter four cylinder received for North American hybrid duty. Both the 3 and 4 cylinder engines are of the same engine family and share the same architecture. It wouldn't be difficult to build the 1.2 three cylinder engine in Dundee, since it also shares many parts with the 1.6 four.
The Fiat 500e could be repurposed into a PHEV using an American built engine and transmission along with other domestic supplier parts to minimize the tariff hit.