In general, the idea of ascension rather than death is only rarely found in Jewish thought. Thus, in Bereishit 5, we find mention of Chachoch:
might even be peshat in the pesukim, given that he lived the same number of years as there are days in a solar year, and given a possible reading of walked with God. The alternative, of course, was that he was a righteous individual, and this was called walking with God. (There is also the pseudopigraphic Enoch who ascends to heaven.)
We see the idea of entering Gan Eden (the source of the four rivers) and living forever for human beings in the epic of Gilgamesh. But even in Bereishit, it would seem that the original plan before eating the fruit of the Tree of Knowledge was that Man would live forever, and God feared (or is alleged to have feared) that Man would become "as one of us."
Besides Eliyahu, we also have Serach bat Asher, who occurs in a late count, and so the suggestion arose that she was extremely long-lived and eventually ascended to heaven alive. Though pashtanim suggest another interpretation of the relevant pesukim.
What about Eliyahu? Did he become an angel who consistently interacts with the world as Hashem's emissary?
One set of relevant pesukim are in Melachim Beis, perek 2. Some excerpts:
intended to describe some form of death, as one does not ascend to heaven except when life is over. Perhaps just as Moshe received Neshika, so did Eliyahu receive this particular form of passing from this world.
There could be a nafka mina in terms of whether Eliyahu haNavi's wife could remarry.
Three interesting points, to my mind:
- This ascension appears to have happened on a spiritual plane, such that Elisha's seeing of anything meant that he would have twice the spirit.
- Elisha cried and tore his clothing, which would typically be considered signs of mourning. This is appropriate for someone's death, but only arguably appropriate for someone becoming an angel. In fact in Moed Katan 26a Resh Lakish objects that Eliyahu is alive, so why the tearing (and why learning about the status of such a tear), and Rabbi Yochanan answers that since he no longer saw him, he was considered as if dead.
- Elisha knows that they can look all they want, but they will not find Eliyahu. It seems that Eliyahu is really taken from this world and is not interacting with it anymore. It would therefore be strange to have Eliyahu interacting within the same generation of even subsequent generations.
Perhaps we can read this figuratively, as a prophet in an Eliyahu HaNavi role, much as Eliyahu on har haKarmel. Or we can read this literally, in which case it is a promise to send Eliyahu haNavi.
If the latter, that tells us at the least how Malachi read the pesukim in Melachim Beis. This may or may not be the same as peshat in Melachim Beis -- though on the other hand, these are words of nevuah.
There is furthermore a difference between interacting only immediately before the yom hashem hagadol vehanorah on the one hand, and continuously interacting with people in multiple incidents, on the other.
In the words of Chazal, we find reference to Eliyahu as interacting with the world, so this conception had taken hold.