Hi @waldgeist,
Complex examples need some context on the problem to solve, so a lot of those will end up emerging via community discussion rather than being general recipes.
However, @Paul_Done, one of MongoDB’s most experienced Solution Architects, has recently released a free e-book covering Practical MongoDB Aggregations with great examples, tips, and cheat sheets:
This e-book is also linked from the Aggregation Reference documentation in the server manual, so hopefully that is a few steps closer to your wish 
.
Regards,
Stennie