There are no known outstanding py3 issues in calibre itself. If you want to help with the py3 port, the place to help is with the plugins. See here
https://plugins.calibre-ebook.com/stats.html to identify popular plugins and here to identify ones that still need porting:
https://www.mobileread.com/forums/sh...d.php?t=326405
Identify one you would like to port from those two links, then check here for the individual plugins' thread:
https://www.mobileread.com/forums/sh...d.php?t=118764
make a post there saying you are porting that plugin and when you are done porting it, post the updated version as a beta for the plugin's users to test.
As for co-ordination with other calibre developers I am pretty much the only developer that contributes to core calibre code in any significant way, so you really need to co-ordinate only with me. If you dont like using the forum you will find my email address at the top of the various source code files in calibre.