This is more of a wish list, but going to post it anyway. Love to see...
I suppose this perhaps is not exactly Web design, but it is a huge part of UX, and I can see this becoming a more prevalent consideration in 2019. While companies such as Auth0 continue to deter creation of new namespaces by making connection with existing ID providers seamless for users and developers alike, exciting new standards like WebAuthn are coming closer to fruition, opening the doors for use of Face ID, Touch ID, U2F (e.g. using a Yubikey), and other future means of "rich authentication."
Moreover, I am a huge fan of the "Magic Link" method where I click a link in an email rather than even having to know or care if I've registered for a service before. See this from time to time, more often each month, expect that to spread like wildfire in 2019, and maybe also expanding to SMS or other channels. All I know is that I did a lot less typing to use the Web in 2018 than I did in 2017. Think this will continue. Maybe Apple will enable Web Push in iOS. Probably not.
Small is the new big. Throughout 2018, sites progressively become easier to read for me. I don't have to turn my head anymore to get through posts. At least on a lot of sites I frequent, I'm able to digest content easier now than before. Likewise, accessibility is becoming more of a prime concern toward the end of 2018 here. I like it when my computer reads to me, so it's always awesome when it knows what to read. Might be dyslexic.
Maybe wishful thinking, but think we'll start to see design trends more conducive to touch navigation on full-size screens (iPad Pro, Surface Pro 6), as that line between tablets and laptops continues to blur. Maybe that's more a 2020 thing, when I think we finally find the middle ground we're looking for, with interfaces that look great and function comfortably on touch devices and desktops alike. Microsoft is doing good work in this space right now.