1. Less Earth More Space
I think to really separate this movie from Avengers it should have less to do with Earth Problems and more to do with outer space problems. This way the movie can establish itself as something different and little unique.
2. Take The Fight To The Villain
Instead of having the villains come to the heroes, the team should ban together and take the fight to the villain’s home world.
3. A Completely New Villain
Obviously people would love to see Darkseid, Mongul or Anti Monitor but I personally think having a new villain one that has not been introduced even in the comics would be an interesting way to go. The audience and the fans would be totally surprised or disappointed/angry depending on how open minded the fans and audience are. It also needs to be a villain worthy of the team someone who can get under the team skin personally and really make an impact.
4. Comic Book Writers/Creators need to be heavily involved.
In order for this movie to really work, people who have been working on these characters and know them like they know the back of their hand, they need to be involved in the writing process and even the producing and directing process. You can’t just put a random person on the project and just expect it to magically be good, it's going to take a ton of time and a lot of effort from everyone to make this movie extraordinary and not just another brick in the wall.
5. Less Dark and Realistic and More Fantastical
This movie needs to feel Epic and grand as out of this world as possible. The parents, kids and everyone should be drawn to this movie and not just one crowd of people. You can have moments of tragedy moments and personal sadness but ultimately don't dwell on these moments.
6. There is no I in Team
Every team member needs to shine, don't just make this a movie surrounding Batman or Superman make it about the entire team. Show us why each member is important and what they can bring to the dance.
7. Make A Great Product
All I ask is that this project is taken seriously and not rushed just to beat out the competition. If you take your time and do it right the first time there won't be a need to reboot or relaunch. Those are just some brief ideas/concepts I'd want in a potential Justice League movie.