Is Millennium Creating A Hellboy Cinematic Universe?
Fans have been calling for another entry into the Hellboy film series ever since Hellboy II: The [...]
Why It Makes Sense
Every major franchise nowadays is creating a connected universe, it's just how things work. Ever since the Marvel Cinematic Universe became a success, all studios are scrambling to find their way into the game.
There are some instances where a connected franchise is the way to go. The DC Extended Universe is a great example. Just like with Marvel, the DC characters are constantly interacting and crossing over in the books. The same should be true for the films.
However, it seems like other studios are grasping for straws at times. A classic monster universe, Hasbro Toy universe, LEGO universe, and Jump Street/Men In Black Universe are all in development.
These don't all sound like great ideas, but they make the studios money. People have a fear of missing out, and they don't want to skip a film that has the potential of tying in to another movie that they actually enjoy. It seems like too much, but it's a brilliant strategy when done correctly.

Where Does Hellboy Fit?
Honestly, Hellboy is one of the few properties that actually warrants a cinematic universe. It's a bit of a surprise that it hasn't happened already.
Over the years, Mike Mignola has created a vast world of characters that exist within the stories of Hellboy. As many saw in the first two movies, the Bureau for Paranormal Research and Defense includes an array of characters besides the title man. Liz Sherman, Abe Sapien, Roger the Homunculus - all of these characters have their own worlds to explore.
Mignola has started to dive into some of these other entities, as he has created spin-off series' based on a few of them. In addition to the Hellboy comics, he also writes books based on the adventures of the BPRD, Abe Sapien, Lobster Johnson, and Sir Edward Grey. These adventures have allowed the creator to grow is universe to the far reaches of the Earth, time, and hell itself.
All of these stories exist within one, coherent universe, and they constantly overlap with one another. It's the perfect set-up for a series of films.

So, Why Replace The Creative Team?
If I connected universe is the goal, why dump the foundation of one that already existed? The first two Hellboy films built up the world of Mignola's characters, so the hard work was already done.
On top of that, those films have the fan-favorite team of Guillermo Del Toro and Ron Perlman. Recently, Del Toro asked fans about their interest in seeing Hellboy 3 come to life, and the response was so overwhelming that the two men sat down for a meeting with Mike Mignola. The creator may have already known about the reboot at that time, because Del Toro sadly announced that Hellboy 3 wouldn't happen.
While no one wanted to hear that Perlman and Del Toro would depart the franchise, it makes the most sense for a connected universe.
Think about those two guys for a second. Del Toro has his hands in countless projects, and it seems like he always has a new movie or TV series on the horizon. The director wouldn't have the time to begin building a brand new universe from scratch. On the other hand, Neil Marshall does have that kind of time, and he's respected enough that everyone trusts his creative ability.

For Perlman, it likely comes down to his age. The actor was a perfect Hellboy, no one doubts that, but he just turned 67 last month. That certainly won't keep him out of work in most of Hollywood, but it does keep him from being a viable lead in a connected universe. Robert Downey Jr. and Chris Evans are already talking about being worn out from making Marvel films, and they are 52 and 36, respectively.
The likely worry is that Perlman couldn't provide the longevity that this franchise is hoping for, and no one wants to replace the lead actor halfway through. David Harbour fixes this problem immediately. The actor is 42-years-old, and his name is currently blowing up around the industry after starring in the ultra-successful, Stranger Things.
A Hellboy universe could be the next big thing in Hollywood, and Millennium is taking the right steps to make sure it lasts. If we're lucky, it will all be as good as it sounds.