Nasu in his interview. There are two options for F/GO after lostbelt ends.
1) Chaldea's story continues from part 2
2) Chaldea's story ends in part 2 and a new story begins.
He personally wants option 1 but it depends on the player base.
Which one do you want?
Ps: FGO ending isn't going to happen. It's too popular.