The outlines of it do seem to already be fairly clear, though there'll always be a couple people who just don't make it expected to be serious candidates (Gillibrand '20 as an excellent example) and a couple of people who come out of nowhere (Buttigieg '20 being another excellent example).
There does seem to be an unusually large number of people who are quite clearly gunning to be the immediate post-Trump nominee and are very plausible: Pence, Haley, Paul, DeSantis, Cruz, Cotton. There are a couple more who might struggle to be taken seriously but are clearly fancy themselves legitimate contenders, like Hurd or Hawley.
See username! Go Haley