I know it's going to have humor and I want to, but Guardians is a comedy which the FF should not be. I also think that the feel of the movie should be different, the environments the FF explore should almost be another character themselves that's how important they are. The FF should be science and exploring unknown focused, like a hybrid of Star Trek, Star Wars, Doctor Who and Indiana Jones, but with a super-powered family.
I think a different director needs to helm this, but I'm not sure who.
I never liked how the movies did Sue's force-fields. They made them look too much like energy-waves when they should look solid and like glass. I think they should not use her internal powers unless the films decide to go the Malice route or a villain in the story calls for her taking drastic action. Here's how I think they're powers in action should be handled:
Ben - The easiest to do. Have him brawl like most other super-strength heroes. The only difference is that his durability should be emphasized as he can really take a beating. Make people want Hulk vs Thing.
Johnny - More focus on his flying and the speed when he flies. It should be on the level of Spider-Man's web swinging and make the audience go "wow". His fight scenes should usually be with him in the air raining fire down on enemies. He can make fire-constructs, so they should be more varied than a basic fireball and flamethrower. His Nova blast should actually be huge blast that shines like a dying star, not like in the first film where it was a lackluster fire vortex.
Reed - The most difficult to pull off. His powers would not be used as often as the others as he usually relies on his inventions. But that doesn't mean his powers should be shied away from like in Fant4stic. I'd say his powers should usually be for support of the others. But when he fights I'd say take some inspiration from the main character in One Piece and uses his rubber powers to up his damage for example a punch would be stronger if stretches his arm far back and then release it. Shouldn't be afraid to make look gross when contorted as he should look slightly gross.
Sue - She's meant to be the most powerful. Show it. Besides having her block things that could kill the entire team, have her be great offensively too. Her powers are only limited by her imagination so that means she is only limited by the script. She can make her force-fields any shape, size and density. She can smash, crush, explode, suffocate, hurtle, smother, eviscerate, pummel and do much more to enemies without them seeing it coming. For the audiences sake we should see the force-fields like glass, but sometime we shouldn't them at all to make it obvious that they too are invisible. She should be their secret weapon. Also, she too should be able to fly on her force-field shaped like a disc. All that stuff is not even mentioning her invisibility powers!
The team - Their teamwork should be the best in the MCU as they know each other so well and the fights should show that.
Here are some panels from comics to helps show what I mean when they use their powers, mainly of Reed and Sue as they suffered the most power-wise in the films:
Sorry in advance for the amount of images.