Anyone familiar with Tomato source-code will be familiar with the structure - heavily influenced by Tomatousb. Haven't checked yet but Asus code usually fully compilable with a little effort.
New features in the source at least, not sure if enabled yet in released firmwares - but already turned on in rmerlin's builds for the RT-N16 I have looked at - include
Dual-Wan, load split or failover (but maybe not recovery?)
Sourcecode includes 64kB nvram CFE sources, possibly only for the latest AC66?
Big thanks to Asus embracing the open-source concept and continuing to support relatively mature routers, and finding a use for the 32MB flash on RT-N16 - firmware now nearly 20MB!