They could just keep one of the coasters (I liked Red a lot better), dramatically shorten the queue, and use the leftover space for other things. That would keep an even split of two major coasters in each park (I'm not counting Flight of the Hippogriff).
A trackless ride would be a good high capacity, all ages ride, and would dovetail nicely with Fantastic Beasts, as mentioned earlier.
As an aside, how long does the DC queue really get? When we when in March, there was only a line from a couple hundred feet before the coaster split. The entire outdoor area and the majority of the castle was empty. We thought it was odd, is that normal?