Is it possible to develop a complex game without programming anything in c++? I mean just working with the blueprint system.
Sure is! Try looking at the Swing Ninja demo on the marketplace.
Ofcorse 
watch this:
Yes, you can.
Great, thanks for all the answers
Well, some networking and deprojecting stuff isn’t completely covered in Blueprint yet, but Epic staff says it will be pretty soon. So by now most of a game’s features, maybe all of them depending on what you are doing, can be achieved with Blueprint solely.
The only networking feature I want to implement is a coop modus and the implementation can wait until it’s supported.