Low Hanging Fruit
Darling still lacks some OS X system calls. Some are easy to implement, some are a lot more complicated.
Darling’s implementation of OS X system calls for Linux resides in src/kernel/emulation/linux. You can see which ones are completely missing by looking into syscalls.c. You can also look for TODOs in existing implementations.
Darling desperately needs a reimplementation of Apple’s CoreCrypto. This library is not open source, although its source code can be downloaded from Apple (see at the bottom). CoreCrypto is needed for any recent version of CommonCrypto and for the Security.framework (which is needed for CMake, codesign etc.).