Hear me out.

We are currently in the middle of the elbaph arc. This is one of the arcs where the story and character is very much focused towards the narrative of D. So my point is that we will get at least basic information or most likely the full name of letter D. Oda has given a bit more focus towards the D during this arc than previous arcs.

At first the elbaph gave us flashbacks of rocks who happened to be D and also share the lineage of Davy jones.

Secondly, we see imu giving us one of the huge statements stating that  "never forget elbaph is of D".

Also the antagonist like Killingham mentions the fear of clan d, nika and gives us rain god lore.

Moreover, the most important part is the presence of saul himself being D and also happened to be residing in the elbaph. 

One of the important details that needs to be noticed is that he had a conversation with Olivia who was researching the void century. Also my assumption is that she could have revealed about D to him. He then turned against WG and also decided to help robin later. Also now we know ohara has been linked with the D clan since recently clover was revealed to have that middle name.

Finally Gabon invited to meet strawhat afterwards. So Gabon together with Saul will reveal to them some secrets related about D to strawhat and especially Nico Robin being the one most curious about void century and D in the crew.

The secret of D will be revealed at the end of elbaph