They are taking the shortcut to the Avengers, unfortunately, I don't think it will have the same pay off since it won't exactly have the same build up.
If this next movie fails to hold it's audience, or even build one on top of Man of Steel, then the universe is effectively dead before it really began.
So hopefully most of these other characters are mostly cameos and/or part of a 5-10 minute post credit short that kinda introduces them for the next movie. A JL Teaser of sorts.
Having an established Superman, a new Batman, a new Lex Luther and possibly 1 other Big Bad (played by failed Conan?) is already a big thing to juggle. A cameo or minor role/cameo by Wonder Woman and/or Nightwing is already jam packing it with introductions to "new" characters. Lots of back stories to fill, personalities to construct and relationships to build.
Only going to be more difficult when you add in Martian Manhunter, Green Lantern, The Flash, Green Arrow and possibly even Aquaman.