One Piece is known for extreme foreshadowing and grandiose world building, which allows for some interesting fan theories. So, let's see what you've got :)

I'll start.

I've always suspected that Imu was from the Void Century or earlier, due to them having history or knowledge of the giant straw hat frozen in impel down, and with the recent chapter it's looking moreso indicated.

My thoughts about how and why he became immortal, are because of the Op Op fruit, but the reasoning I think is because it was necessary to beat Joy Boy the first time around.

Joy Boy, or at least Luffy's manifestation of it, seems to sap vitality whenever it's used. My suspicion is that the Op Op fruit is necessary in order to achieve a full form Joy Boy, or else Joy Boy will just die of old age. So, Imu used it in order to steal that opportunity from Joy Boy in the past.

So, if I had to guess, Law will sacrifice himself for Luffy toward the end of the show to allow Luffy to permanently enter the Joy Boy state and bring eternal freedom to everyone!