I've found on a forum some new info about the future version of Xbox Live. Since I do not take infos found there for granted I asked a friend who works for a game developper for confirmation, and he confirmed everything.
These features require release 3.0 of the Xbox Live service. The Xbox Live 3.0 release is sometimes referred to as "Tsunami". The Xbox group is releasing Xbox Live 3.0 around the beginning of April 2004. Titles that take advantage of these new features should schedule their releases to occur after the release of Xbox Live 3.0. Titles that intend to release before the release of Xbox Live 3.0 should not use any these features.
Voice Mail in UIX Friends : The Voice Mail feature allows users to include voice mail when they send or accept game invitations and friend requests. Users can include up to 15 seconds of voice data with each invitation or request to provide more context for the recipient.
Title Managed Storage : Title Managed Storage allows a game to store information on the Xbox Live storage servers. The feature is divided into two parts: global storage and per-user, per-title storage. Global storage is read-only to the title and stores matchmaking queries, rosters, weather data, and so on. Per-user, per-title storage provides 32 KB of storage for each player of the title, to store playbooks, settings, and so on.
Family Title ID : Family Title IDs allow title-managed storage, teams, and messages to be shared across a family of titles. A given title can belong to as many as four title families. The way in which storage, teams, and messages are shared across the title family is up to the publisher and title developers. For example, a publisher with multiple sports titles could have a developer use a Family Title ID that allows a user to create a team that spans all of the publisher's sports titles.
Teams : The Teams feature allows players to form teams within a title that persist between game sessions. Titles expose this functionality using the XOnlineTeamXXXX and XOnlinePeerXXXX functions. The infrastructure, permissions, display, and role of teams depend on the design of the title.
Competitions : Titles can use the Competitions feature to run automated in-game single-elimination tournaments. The participants in a tournament can be either teams or individuals. Tournaments are created either by the title publisher or by the players themselves.
Messaging : With Messaging, titles can send and receive any kind of game-specific message (including voice and text attachments) using the XOnlineMessageXXXX functions. Messages can be sent between teammates, friends, or players.
New Xbox Live Statistics : Features The updated Stats APIs enable titles to group users into dynamic teams, called "units". Each unit can contain up to four players. Users can compare how their units rank against each other on the new Unit Leaderboards. Also, titles are now able to resolve contested stats using the Xbox Live Arbitration servers. Additionally, titles can now post all of the stats for a game session with a single write call.