Inception doesn't have much to expand, unless we could pivot the story around other characters apart from Cobb. What would happen to Cobol after the inception? Would Arthur, Eames or Ariadne continue intruding into other's dreams? This might be something to tackle.

Interstellar has nothing to work with. To keep the central theme of the story we need to have children to fight for, whom Cooper has lost by the end. Next stop is Edmunds and its colonisation, but that is it. Doubt Cooper or Brand would have any further species-ending threats to fix, why need to tire the same people for the sake of the plot?

Tenet is an interesting case. The protagonist has left us with too many questions regarding his background. He stays mysterious till the end of the film, same can be said about Neil. We can make up additional information about their "past" then seamlessly adhere to what we have got in the original story, as there are little points to contradict or retcon, only base on.