I had a very similar problem with this film as I did with Superman. There’s some good ideas here, but by foregoing the origin story and jumping us ahead years after they’ve inherited their title/powers, something is missing.
I know the Fantastic Four. I know Superman. I know the supporting characters. But I don’t know THESE renditions. And yeah you can have the basis there and a quick montage or voiceover of “how we got here”, but watching the banter and chemistry the characters have isn’t enough to make me connect with them emotionally. In the comics, a lot of the heart and soul of a superhero is formed during their early years. You can callback to their trials and tribulations verbally or indirectly, but it’s not enough.
That said, I thought the movie was fine. Entertaining enough and I’m glad it at least tried to be different than what’s come before in the MCU. But as a launchpad for the future linchpin of the universe moving forward? This didn’t even get close to making me excited about their entrance into the Avengers.