My idea is to combine as much of the Swiss Scenic Railroad as possible with a few days in Venice. We've taken the Golden Pass, but otherwise... it all looks fantastic!
I'm thinking about the Glacier Express from Zermatt to the Bernina Express via Chur... which would take us into Italy... spending a few days in Venice.
Then maybe back up into Switzerland via the Gotthard Express which starts in Lugano and goes through Andermatt, to Lucerne -- which is only about 45 minutes from the Zurich airport.
We live in the Boston area, so have the option of a direct flight in and out of Zurich -- though open to open-jaw route if it's the best option. I can see that there's no direct or simple way to get from Zurich to Zermatt to start the Golden Pass, but as Switzerland is so small it may not be an issue...