I recommend A Windsor's
Pirate Children\'s series. Wonderfully written stories of Mal and Inara that later centers around their two children.
There's also
Stay that I found to be extremely well-written.
Lastly, anything HawkMoth writes is worth reading. Since you're a Mal fan, you should read his story "
Bloodlines ." It's a story about where Malcolm Reynold's came from.