In the last set of official Commander decks, a number of the never-before-seen cards had a distinct multiplayer focus. Among them was a horizontal cycle of rare cards linked by an ability word called join forces, which essentially allows every player to contribute to an X-mana cost. Since this is a good, if unsubtle, way to encourage interesting politics around the table, I suspect we'll see another cycle of join forces cards in this year's Commander product as well - except this time, they may be multicolor. So how might they work? Let's take some guesses:
Green-white: Inasmuch as join forces inherently has "group hug" applications, the green-white member of the cycle would be the friendliest of them all. This would be a big boost, but most relevant to decks that run a lot of creatures at once - such as, say, the typical green-white aggro deck.
Triumphal Festival 3GW
Join forces - At the beginning of your upkeep, each player starting with you may pay any amount of mana. Put X +1/+1 counters on each creature, where X is the total amount of mana paid in this way.
White-blue: Admittedly, this is the hardest one to figure out. Maybe we'll get something more along the lines of Mana-Charged Dragon here; it's very like white-blue to appoint itself the game's police officer and go after troublemaking players with the support of the others.
Thoughtweft Shackler WU
Creature - Kithkin Knight (Rare)
Join forces - Whenever Thoughtweft Shackler attacks or blocks, each player starting with you may pay any amount of mana. Thoughtweft Shackler gains protection from converted mana cost X or less until end of turn, where X is the total amount of mana paid in this way.
Blue-black: Shared Trauma already covers the typical mill effect that this color combination likes to use. No matter; there's no shortage of devious things for blue-black to do with the help of its partners in crime.
All Our Sins Remembered 1UB
Join forces - Starting with you, each player may pay any amount of mana. Each player returns any number of target instant or sorcery cards with total converted mana cost X or less from his or her graveyard to his or her hand, where X is the total amount of mana paid in this way. Exile All Our Sins Remembered.
Black-red: Who says the cost in a join forces ability has to be mana? Black-red is the perfect color combination to reverse the typical version of join forces, adding to each player's mana pool but requiring a gift of their life totals. Making this balanced would be difficult, but that could be easily accomplished by allowing opponents to have the benefits first.
Ritual of the Coven 3BR
Join forces - Starting with you, each player may pay any amount of life. At the beginning of the first main phase of each player's next turn, that player adds X to his or her mana pool, where X is the total amount of life paid in this way.
Red-green: Among the original join forces cards, Shared Trauma is the only one that really dishes out pain to everyone. But sometimes a little mass destruction is cathartic. Who cares if it's stereotypical to the color combination - it's sometimes what a crowded EDH table needs.
Critical Mass 4RG
Join forces - Starting with you, each player may pay any amount of mana. Each player sacrifices X permanents, where X is the total amount of mana paid in this way.