This one is pretty large compared to any other issues, it entails the implementation of the following handlers:

POST /web/osu-osz2-bmsubmit-post.php
POST /web/osu-osz2-bmsubmit-upload.php
GET /web/osu-osz2-bmsubmit-getid.php
GET /web/osu-get-beatmap-topic.php

along with writing a program to write osz2 files, and quite a few backend changes.
I've already done a few hundred lines of code of this one, but it will likely not be implemented into master for a while to come.