Using Anime logic it is a sequel or a spinoff in my opinion. JLA is one of the best animated shows of its type from cable tv. My opinion is that it feels and looks alot like the Batman TAS series and it even has some characters crossover, if you will.
There has been several episodes that featured the supporting cast, such as the one about Martian Manhunter or the Green Latern Corps or the WWII one featuring Wonder Woman and Superman in the future. It is a team concept show but Batman always stands out as the main eventer.