yet some have already been implemented .. like your central contact database is allready part of the zarafa groupware suite that is included in zentyals main track so there isn't any need to add it anywhere else is there?
I know, I'm late but if I don't post it now somewhere, I'll forget it as usual
The built-in central contact database for Zarafa(!) is available but need to be extend it to use with email client (I guess this is the main use). Now, it's possible only with lot of tricks