Those portable HDMI screens would work just as well on a 2016-2018 (no dongle required) - cables that convert the USB-C/TB3 port to HDMI are a dime a dozen, and support at least 4K resolution. I'm not sure if they're technically USB-C or TB3 (will they work on a MacBook or a USB-C Windows laptop)? Either way, they work fine on MBPs - I have two of them, one driving a monitor and a long one in my projector case).
As for "will the new 16" replace the 15" or be a new premium model?", my guess is that the new machine will be the new primary model, but that Apple may leave the lowest end model of the current 15" design in the lineup, starting the new model above it. They often do this at a redesign.
Can you recommend a model? I have two in my watchlist on Amazon but they are sold by someone that mainly sells clothing and I'm a bit wary about buying from them. My current case is that I like the 2015 rMBP but there are times when I'd like more screen real estate. My solution is to carry two 15 inch rMBPs but an additional portable display might be an answer. The advantage of two rMBPs, though, is that I can use both screens when I can't get a table near a power outlet. The portable HDMI screens that I've seen are USB-C powered and I assume that you need a wall plug.