The title says it all.
Where is the most profitable Business within the future preferable a prediction within the next 10 years. Please don't say oil or retail stores like Target or Walmart because they cause a lot of problems.

Side Question: How much would it cost to build a solar plant to power a city including costs to maintain it? (So a solar powered city instead of oil) Please ignore predictions of prices for roads, cables, work quarters(though include wages of workers would will maintain it) and assume plant is in the desert next to city to minimize costs. Estimated size of plant and cost under these conditons would be greatly appreciated!
Thank you for your insight =P