(Note: I haven't finished this post yet, I posted it here unfinished just to remind myself that I have to finish this post one day.)

The database idea I have in my mind now is:

A database system that is mainly used for logging, let's call this imaginery database system as LGDB for now. LGDB does not have to fully support ACID. It is some kind like the ARCHIVE engine(1) for MySQL, but optimzed for speed instead of compactness. And, there are some functionalities that I wish that I can see in this system:

a) High Thoughput: Imagine a logging server that runs LGDB behind a cluster of web servers. Each web server will have to serve its logging stream to this logging server. And, that's why LGDB has to be fast.
b) To be continued.

1. http://dev.mysql.com/tech-resources/articles/storage-engine.html