I envision 2 possibilities for threads.

1. interactive mode.

$ x thread

Hello world

If you press enter it prompts you:

Press `space` to continue with 1/1 and add a new line
Press `enter` to continue with 2/2
Press `shift + enter` to send thread
Press `esc` to restart 1/1

I'd replace the -t command with this functionality and just tweet without any param like currently does but it's not documented btw.

May be feasible to implement color highlight when going above max char.

2. Reply to last tweet.
   x -r "text"

So you can:
x "Hello"
x -r "world!"

Hello tweet 1/2
world! tweet 2/2

3. Reply to id
   x -r [id] "text"

Which can be combined with
x "Hello world!" ~> outputing the ID