Well, it's hard to disagree that Batman V Superman was an extremely divisive film, but it's harder to disagree that not many people were disappointed by BvS's ending. Indeed it did come out as a slight surprise that Superman dies in the introductory movie itself, but it wasn't a shocker. Though I wasn't expecting that Snyder would use the story-line of 'The Death of Superman' so early in the very initial stages itself, but as a whole, since majority of people were against Superman in BvS it was pivotal for Superman to die (I will explain that in a while)
Well the second major aspect of the ending was that Luthor states that the 'bell has rung and he is coming'...but who is that "HE"? Let's separately analyze the ending:
Superman seemingly dies..
As I said earlier since a major portion of the people were dead against Superman and they believed that he was supposedly a threat to all of humanity, it was pivotal for Superman to sacrifice his life and present to them the fact that he is indeed trustworthy. If you are a comic-book acolyte you'd have by now guessed this for sure. But then the question arises that how would you bring him back to life?
As I said 'Superman seemingly dies', because there is a chance that he actually ain't dead (literally). There are many ways that Zack could bring Superman "back to life", one of them being with the help of S.T.A.R Labs. But as of now in the story-line Bruce doesn't know anything about the S.T.A.R Labs. Only time will tell how Zack proposes of bringing him back to life, but he sure will rise.
What Luthor meant..
The "HE" Luthor mentions is pretty well obvious is 'Darkseid'. Quite early in the film, while pointing at a painting, Luthor quite dramatically mentions that devils don't come from beneath us, but they come from the sky above clearly indicating Superman (The red capes are coming), but one of the final shots of the movie i.e. right after Luthor tells Batman that Darkseid is coming. For the record, Darkseid is known as the "Devil from the sky".
Well we know that Darkseid is gonna be the supreme antagonist in the Justice League movies, but I feel that he would only come in by the 'Justice League 2' because 'Justice League 1' would be more inclined towards bringing the members of the Justice League together. Another thing that Justice League would have to answer is 'What did Flash mean when he said to Bruce to find them and that Lois is the key?' Well when will Superman come back to life is not known either, because first there is many things that has to go down before that (the formation of Justice League for instance)
Well only time will tell what the DCEU has in store for us but guessing doesn't harm, does it?