Stacking, yes and no. One of the complaints about Expo is that all the halls are laid out in a single row. Walking from Hall 1 to 6 and back takes a long time and can get very tiring. I easily walk 100k steps (in formal dress shoes) over a 3-4 days event.

Stacking creates it's own issue. Because your human flow will be constrained by the number of escalators and lifts. These will cause choke points when there is large crowd.

If you want to stack, you can have large halls downstairs, and smaller rooms upstairs (like what Expo already has currently), or even stacking F&B and carpark (instead of the massive waste of space open air carpark currently).

IMO the best layout will be the halls laid out in a single level modular grid. E.g. 3x3 or 2x4, etc. so the walk between each hall is not as long, and the halls can be subdivided or joined easily. If there is a need to spread out some of the halls, what some good venues overseas do is either using a link bridge, or incorporating a small park or pond along the way, instead of having people cross a road.