They don't need
to be in a shared universe but it is a nice bonus
From the way I see it, the proper way to do a shared universe is by focusing on each individual superhero and his/her part of the universe with no sort of Avengers or Justice League setups whatsoever. The solo movies should come first and be their own stories while the big superhero team-up should be the big bonus at the end if all of the solo films are successful.
I blame Marvel for creating the idea that you can't have a shared universe with self-contained stories and that shared universes somehow limit originality. This is false. A shared universe simply means these stories exist in the same universe. There is no reason why you can't have a Batman or Superman movie that focuses just on their respective side of the DC mythos but still take place in the same universe. Technically speaking, all 3 stories that occurred in the Nolan films could have occurred in a shared universe (and I'm talking about just
the stories here - I'm ignoring the realism). You wouldn't really have to do any major changes other than discarding the 8 year gap and the ending of TDKR.
I really hope WB realizes this and doesn't just turn all their solo films into JL setups. They will have a far
superior universe to Marvel's if they put the solos before JL, let them tell a whole story as opposed to turning them into JL promos, and then have the big JL film as the big bonus at the end.