[Finwe]

Eru told Morgoth in the Ainulindalë that all things had their uttermost source in him (Eru), and that whatever he did, however he tried to rebel, everything would end up following the will of Eru. That same principle can be applied to the rest of Middle-earth.

I think that Eru's "plan" was to give the Peoples of Middle-earth a pretty long rein, but still keep an "eye" on them. Their fates were determined (for the most part) by their actions, except for characters like Luthien, Beren, etc., whose fates were essential to Middle-earth's history. Even then, for example, had Thingol agreed to the union of Beren and Luthien, without asking for the Silmaril, Morgoth would have still had all three Jewels, and Earendil probably wouldn't have been able to sail West to ask for the aid of the Valar. Granted, when Morgoth did eventually conquer Beleriand, the Valar might have sent the Host of the West anyway, but it was through Earendil and the Silmaril that Beleriand was saved.

To state my opinion in another way, I think Eru wanted the fates of the people of Middle-earth to be a kind of interpretive dance (rather like the dancing that one does on a dance floor), instead of a rigid, structured, choreographed dance (like ballet or other "performing" dances). He set some loose guidelines for his Children, and then sat back to watch what they did with those guidelines, and what they did with the choices given to them.