First, I should note that in this scenario....Trump has to win the election, and this isn't a guaranteed thing.
Second, the Army has put a fair amount of money into the installations of the Wiesbaden area, and for dependents....this is a four-star tour. Where ever the placement of troops were to occur, it'd take several years to build and accommodate these troops.
So, in my fantasy scenario, even if a decision were reached by the fall of 2017 on some new closure plan around Europe, and Wiesbaden ended up on the closure list.....I doubt that the soonest that such an event could occur is 2021. You can also figure that with various inspections and delays built into the system....that the "keys" to the gates and facilities would not be handed over to the city of Wiesbaden until the end of 2024. The city planners would probably consume an entire year (even if they had five years of prior notice) before they could reach a decent reuse plan.
You wouldn't see anything done prior to 2026 (a decade from now).
I should also note that another city election will occur by 2021 (with a second election by 2026), and if the immigration issue is still on the front burner, it's likely that the AfD could take double the number of voters that they had from three weeks ago. That would heavily influence the planning side of this.
There are four significant housing areas in the Army footprint of Wiesbaden. I think the city would review their options, and likely keep them as they are. The newer housing units out on the airfield might even go as condo type units and be sold.
I think most of the hillside facing the city on the Hainerberg 'hill' would be torn down. Some city park would likely come out of this, and being next to the soccer arena....some of it might be used for parking.
The new school being erected on Hainerberg? I'm guessing that it'll remain and some older city school in a bad state of operation would be shut down, with students moved to this new site.
The runway area? That's the curious thing, and likely to draw a lot of comments and criticism. Frankly, it'll be hard to fight off the Frankfurt Airport folks from desiring the property.....all of it.
The runway is roughly 6,200 feet long. That's ok for medium and smaller aircraft. Enlarging the runway? You can only do this by going in the westward direction, and you could easily add 1,500 feet onto the runway, to make it large enough for major passenger jets. But you'd have to do something about the houses in the Fort Biehler area (roughly 40 homes). Maybe if the Flughafen folks offered thirty-percent above the going rate for these homes.....a number of these people would be happy to accept and leave. You are probably looking at 20-million Euro to buy and then dismantle the homes. You'd also have to build some tunnel device for traffic on B455.
Running some tram from the Frankfurt Flughafen out to the Wiesbaden site? It'd likely take two years to build it along the autobahn, a 24-kilometer route. It'd probably be 15-minute shuttle ride to go from X to Y....free of charge.
Acceptance from around the Wiesbaden community? No. I'd take a guess that sixty percent would have issues with this and it'd be a political issue dividing the area. Some would hate the idea of the active runway connected to the Frankfurt Flughafen. Some would hate creating mini villages that was designed more for refugee or immigrant housing. Some would hate some commercial use talk over the Hainerberg area.
A different Wiesbaden in ten years? Yes.
Army personnel don't count toward the two-hundred-and-eighty-odd thousand residents of the city.....which they like to brag on population charts.
If you removed the Americans from the equation, had the facilities, housing, and runway area....you can figure the population growth would be at least another ten to fifteen thousand that would occur (especially if the runway were reused). The city was already planning on another five thousand residents over the next five-to-ten years (the private university and immigration would help). So you could be looking at a city of 300,000 by 2030 very easily. Considering where they were in 1800 (2,239 residents in the sleepy-eyed valley of Wiesbaden).....things have radically changed.
So, in a fantasy world, where Trump might win, and some downsizing plan did occur.....this is where I'd see things going.