[Perception is everything in poetry: we each interpret a poem based on our own life experiences, knowledge and ideas. This is my personal reaction.]
You have identified several of the characteristics of Romantic writing, including individualism, inspiration, idealism and imagination. Emily Dickinson includes quite a few Romantic characteristics in this poem.
We might define individualism as...
...a single human being, as distinguished from a group.
Taking this definition quite literally, the speaker is the only human in the carriage.
The other two occupants are examples of the Dickinson's imagination; with the human passenger are the "supernatural" manifestations of Death and Immortality.
In studying the content of the poem, Dickinson is inspired to present Death as a suitor, one who woos for love:
He kindly stopped for me –...
...For His Civility –
Immortality is present to make sure nothing improper takes place:
Immortality also rides in the carriage...[because of] the necessity of a chaperon.
It is not until the end of the poem, that the speaker realizes her "gentlemanly companion" has actually carried her off to eternity.
Death is in reality a ghoulish seducer.
Idealism is probably best seen in how Dickinson imagines death: she is courteously "collected" and carried off in a proper way, in a proper carriage with a chaperon. She leaves her worries behind and glimpses, through the window as she passes, children at play and the beauties of nature—scenes of peace and comfort.
We passed the School, where Children strove
At Recess – in the Ring –
We passed the Fields of Gazing Grain –
We passed the Setting Sun –
There is no need for worry, as seen in the casual pace of the horses:
We slowly drove – He knew no haste...
Death is not frightening; there is nothing to cause fear. All is tranquil and seemingly innocuous.
Adventures in English Literature, Harcourt Brace Jovanovich Publishers: Orlando, 1985.