Hi @CekLive. Thanks for posting. Although I do not understand what ideas have been discarded, given you are a first time poster.
if K9Mail really should become Thunderbird
K9 still exists, but now so does Thunderbird for Android Download — Thunderbird
So you are looking for what is called a hierarchical view of the folders? Can you post your wish at Ideas - Mozilla Connect if it’s not already there. It will be a great help, because we are using Connect to collect ideas for future development, and for user voting. Also please add iIn what way do you feel that would help the UI?