In the long term, millenials retiring is going to cause the market to fall, because there is going to be significantly fewer workers to drive the market. We kinda lucked out with millennials being the shadow boom that takes the spot of retiring baby boomers, but there doesn't seem to be another generation that is as large as those two. Less workers means lower stocks.
In the short to medium term, internet advertising rates are crashing right now. Only google and facebook are doing ok. That is not great news for venture capitalists in Silicon Valley. Combine that with, say, Amazon seeing only modest growth and all of the sudden the two drivers for real estate value in the West Coast are gone. Yay, another boom/bust in the housing market.
In the medium term, people are getting more urbanized and it's easier to get around without your car. I see a lot less cars being bought, which means more money in real estate and restaurants, etc- but the car manufacturers are on the stock exchange and those aren't.