Besides working on allowing developers to build native, full-speed
apps for the Apple Watch, Apple is working on allowing third-party
watch face “Complications,” according to our source. Complications
are the small widgets indicating activity levels, battery life,
alarm clocks, upcoming calendar events, and the current
temperature on many of Apple’s included Apple Watch clock faces.
Our source says that Apple is currently testing a new version of
Watch OS that notably includes a set of Twitter Complications. For
example, a small Complication could display a count of unread
Twitter mentions, while a larger view could show the text of a
recent Twitter mention.
Just based on my own gut feeling, third-party complications feel a lot more likely than third-party watch faces.