Based on the statements of numerous characters following his death, it seems fairly obvious as to whom John Locke will contact during his days as Jeremy Bentham, and what such discussions will constitute. The following is a theory of how these will be revealed to us, the audience. It is not based on any spoilers, but rather is stated in a factual manner purely out of stylistic preference.
When he first leaves the Island, Locke will proceed immediately to Eloise Hawking, per his instructions from Christian Shephard. She will reiterate the need to convince the Ajira Five to return, explaining that Aaron’s fate lies elsewhere, as does Walt’s, for the time being at least. At this point her relation to Widmore will become clearer, with her prompting John to confront him as the first stage of his quest.
Widmore shall act as a tempter, playing to John’s dedication to protecting the Island as well as John’s own self knowledge of being “chosen” for that task. After struggling with a moment of doubt, John will reaffirm his allegiances, rejecting Widmore in an initial moment of triumph.
Going next to Walt, he shall learn that the boy has no recollection of giving instructions to John while after he was shot and left for dead by Ben. Locke will deliver Ms. Hawking’s message and leave.
He will then contact the members of the Oceanic Six, excluding Sun, out of respect for Jin’s wishes, and made evident in that she made no indication of learning Jin’s body washed ashore when presented with his wedding ring by Ben. However, much to his surprise and grief, Hurley, Sayid, and Kate will all refuse him.
These visitations will culminate with Jack, who will fail to believe him for the final time, symbolizing the Six’s ultimate rejection of the one man genuinely offering them a chance at redemption. This sin is what shall necessitate his death, as he will have become the sacrifice the Island demands on their behalf.
As he proceeds to hang himself, partially as a last ditch effort to unite the five through this sacrificial act, and partially out of the despair for his apparent failure, he himself will be visited by Ben. After giving him Jin’s ring and explaining what led him to this, Locke will reaffirm his faith, and in doing so, die.
All this shall be shown as a flashback experienced by a fully resuscitated Locke, having emerged from his coffin to find himself on the Island in 2008, with Ajira 316 survivors looking to him for leadership.