It would be kool having at disposition a "chat like" service incuding only a contact list acces while playing, and a possibility to send a message to someone even if he's disconnected. I know it is probably a bit hard to have so much data-space at disposal, but it would personaly be a great step increasing the social assistance of this already great game.

Use the /buddy command set.

/buddy - displays a list of commands
/buddy invite "character name" - Invites "character name" to become your buddy.
/buddy accept "character name" - When you've been invited to be someone's buddy, this makes you their buddy.
/buddy decline "character name" - This declines the offer for someone else to be your buddy.
/buddy remove "character name" - Removes "character name" from your buddy list. You are removed from theirs as well.
/buddy list - Lists all of your buddies.
/buddy online - Lists all buddies that are ingame.
/buddy last "character name" - Prints out the last time that a buddy was ingame.
/buddy notify [on|off] - Turns on and off the notification of buddies logging in and out of the game. Does not affect whether you are shown or not on their end.

You're missing
/buddy note "char name" message
smartypants

As does the wiki, it appears.

(Edit: Added to the wiki.)