Leader keys should be added with all it's additional configs . Strict Key Processing , Infinite Leader key timeout, Per Key Timing on Leader keys , leader time out. leader_start() and leader_end() could be nice to haves if this is hard to implement.
Suggestions on what the output of a leader key could be
Single keycodes (simple and advanced)
sendstring (without needing to create a macro, if possible)
UI wise, it could be similar the combos tap in the GUI currently except to make it expandable or retractable based on how many keys. And perhaps optional leader start and leader end boxes