Walmart has already started doing the 2 day delivery thing a while ago for free (no "prime" needed) for orders above the purchase threshold. They then started allowing you to order online and pick up in stored (including groceries).
I think we are going to see Walmart start competing with Amazon even harder. And here's the thing most people don't even realize - Amazon's main business unit dealing with retail sales loses a metric shit ton of money. Amazon just released it's latest quarterly report and somehow people thought it was a good report. It wasn't. Sure, Cloud services are doing well. But Amazon as an entire company makes only a 1.3% profit margin. That includes the high margin cloud business. The actual retail business? It loses 18%!!! That loss is up from 15% the previous quarter. In 2013 those losses were 2-4%! Amazon is getting absolutely ravaged in their retail sales business. Walmart, on the other hand, has margins of about 2.35% and pretty much all they do is retail business. And let's not forget that Walmart has massive brick and mortar stores all around the country and they can still turn a profit. Amazon, who has at this time nearly 0 physical retail spaces (other than a few test/trial stores) manages to piss away nearly 1 in every 5 dollars it takes in from their retail unit.
I sincerely think that Walmart is going to figure out a way in the next 5-10 years to utterly decimate Amazon's main line of business.