I think the plugin / theme developer mailing list is a great idea. Finding out what might affect my plugins is currently a little bit of hit and miss, and this list would solve that.

Not sure about the plugin adoption idea. It's a great idea if the plugin author actually chooses to put the plugin up for adoption, but I'm a little uncomfortable with the idea of doing it without their explicit approval (ie if you can't get in contact with them).

It's all open source, so anyone can take the code and create a new plugin based on it, but actually transferring the original plugin to someone else without permission seems wrong.