With the 'Modern Broly', where does it leave Classic Broly in the franchise as a whole?

[KBABZ]

Will this movie even be kanon or like a filler movie like the Dragon Ball Z movies were?

At the moment it's considered to be. The story directly references the state of the world in the aftermath of the Tournament of Power and why Goku wants to keep training, plus it once again has the direct involvement of Toriyama in the creation of the story, as opposed to character designs like almost all of the 90s movies. Unlike those tales, Super Broly makes an effort to fit properly into where the story is at the moment.

[ShadowBardock89]

If there is another series that is made after this movie and the production team use the plot as an arc (I hate this trend, by the way), consider Broly (Super) what Broly is going to be from now on.
Of course, Broly (Z) is still going to be used in video games, toys, etc. because of nostalgia.