[gluggymug]

    >- jump to previous commands.
    >You just ran a command that printed a few pages, you want to scan from the first line.

I think tmux might get you there. You bind a key to enter copy-mode and backward search for your particular prompt.

    >- collapse previous outputs (stderr / stdout / both)
    > Let me toggle stdout / stderr after the command has been run.

That sounds like a good one.

    >- connect a pipe to a previously run command
    >... But why can't you re-use the output that is there in the terminals scrollback buffer?

Once again tmux could "save-buffer" to a file for you if you did that backwards search then copy to a paste buffer. Just bind another key.