I thought about Mecca and Medina. The haj visitors are already part of their economy, but they need to expand their economic base. I agree those cities would be big draws particularly in the years after opening them up, but that allure would fade quickly. Their best bet might be becoming a major financial center like Dubai, HK, and Singapore.
Dubai, HK, and Singapore are also world-class, business friendly free trade zones, as well as financial centers.
The Saudis definitely need the kind of business policies that have been so successful in Dubai. As it is, if you do business in the region, you almost have to do it from Dubai, because the advantages are so overwhelming.
If you want to open a business in Dubai, you can get incorporated same day, and the Government will drive you around to get everything you need, like a butler. They have offices and warehouses sitting empty, ready for occupancy, furnished, with dial tone on the phones and internet connected - favorable tax treatment, and reliable business dispute resolution in the courts - hard to beat.
Apparently, Prince Mohammad bin Salman in Saudi Arabia is a keen student of Sheik Mo in Dubai - a regular protege. There are some diversification options just over the Saudi borders that could be leveraged, but the business practices of the UAE with Dubai is without doubt the crown jewel.
Yes, the huge craters where they once stood would be an incredible testimony to the unthinkable horror that once plagued the world....sort of like the museums that were the Nazi death camps.