I would presume that Marcus is dead and that this will be mentioned, and the character paid tribute to in some small way. I don't see Indy becoming Marcus, though.
And I don't think this new adventure will be presented to him as a last gasp. I think he will have remained plenty active with the adventuring in the intervening years, just with different motivations than he had in his early years.
We'll probably catch up to him right in the middle of one of those adventures, as we have in the previous films. Adventure has a name, and that's Indiana Jones. Adventure is his life.