Hi there - I see you're Mr Kafka :-) I wanted to use Kafka instead of Raft for my project, but for my application I couldn't tolerate the (even highly unlikely) possibility of losing a message (when we lose all the nodes in the In-Sync Replica set). I understand why this tradeoff is there (for the clickstream use-case), but I hope it might be possible to retrofit reliable writes to support other use cases as well. The more open-source reliable log services we have the better!
I'd love to help (I opened KAFKA-1050 on the issue), and I even started coding, but I got stuck on how to deal with the rollback problem when a write isn't acknowledged by a quorum. I think it requires a "real" distributed consensus protocol (unlike the current design), which is a fairly big change! It may be that the answer is to use Kafka's efficient storage design with Raft, which is something I explored a little on day 1 of my project: http://blog.justinsb.com/blog/2013/12/07/cloudata-day-1/
