I suspect we are getting closer to a simulator such as used in aviation, rather than an expansive arena maybe just large stall with full motion flooring and surround video sort of like using Google Street view, 360 degrees perspective (and a judge who always awards at least a 70) …of course a real horse is to be ridden rather than simulation (this is not Star Trek). There are already the treadmills for straight line exercising, just need to add more movement, maybe get Elon Musk involved
I think our Bonnie horse would like one of those fantasy arenas, she is such a goofball anyway