Highwoods should be able to build whatever they want on their own property.
New Plaza structures should blend in with the existing architecture and not be too much of a traffic nuisance. Neighborhood pressure is enough to keep Highwoods in line.
Kansas City should create protective legislation for the Plaza that would require Highwoods to preserve the ambiance and protect this unique landmark. (See Rowland's Brookside Business District, a zoning overlay, as an example.)