The main risk for Google, Apple and Amazon is regulations outside the US. I don't know how reliant they are on the US market, where they are safe, but abroad what we saw today is going to keep on happening. The US is too dominant and countries will find any reasons (sometimes valid ones) to restrain them.
That's why I would look at expansion in the US as the source of their future growth, and that would come from expanding in other sectors like energy, telecommunications, healthcare, possibly education, transportation. Housing too ideally, but that seems like a really tough nut to crack.
The car side of things wouldn't make much sense usually, especially for a company like Apple to get into, but I think all three companies have come to the conclusion that if they don't own the platform they'll get screwed, so they will likely have to eventually have their own cars. Otherwise, they'll need a really solid partnership with a car company. If a big buyout eventually happens, I think it will be because of self-driving cars.
Apple will face increasing competition from China. It's the one company that looks like it has the most trouble going forward with expansion across other sectors. The Chinese phones will hurt them outside the US.
