Autofill APIs will allow users to select a designated autofill app, "similar to the way they select a keyboard app." This should make password managers work a lot better, without the need for janky copy/paste solutions or hacks to the keyboard framework.
If the API is implemented properly, this could be great. Obviously, there are associated security risks, but hopefully they'll leave authentication to the password manager devs. Requiring an action to trigger the autofill would be a good first step to prevent malicious websites from harvesting your logins.