It looks like the Tom Cruise-led Mummy reboot may be all that we'll get from Universal's attempt at an MCU-style cinematic universe. According to The Hollywood Reporter, Alex Kurtzman and Chris Morgan, who were in charge of steering what was known as the "Dark Universe," have departed the project.
This isn't terribly surprising; it was pretty unanimously agreed that announcing an entire cinematic universe to be in development before even one movie in said universe was released was a bad idea, and The Mummy was both a critical and commercial flop. Among other critiques, it was widely panned for feeling too much like a setup for a variety of sequels and spin-offs instead of a story that could stand on its own.
As for what could've been: The Mummy was supposed to be audiences' first step into a larger world of classic Universal Studios monsters like Dracula and the Creature From the Black Lagoon. Johnny Depp (Pirates of the Caribbean) was set to play The Invisible Man, Javier Bardem (No Country For Old Men) was to play Frankenstein's monster, and even Dwayne 'The Rock' Johnson (The Fate of the Furious) was rumored to be in talks for The Wolfman.
More and more creatures would have been interwoven into an overarching narrative, much like Disney's MCU or Warner Bros DCEU. THR reports that Universal is weighing options on how to salvage the Dark Universe (or indeed if it should be salvaged) but with the departure of Kurtzman and Morgan and the disastrous performance of The Mummy, it looks like the end may have come for these monsters before they could really start rampaging.