Westeros: A Game of Thrones.

[Beskar]

Admittedly, there are tons of possibilities of what could occur. But having major rivals become allies would be so unbelievably gamey, it would be untrue. These would be for example, Stannis or Renly willingly working with the Lannisters.

Starks to ally the Lannisters... it would be... "extremely difficult" to do this convincingly without it looking completely unrealistic of the situation. During this point of time, I believe Ned is trapped and he just tried to send a letter to Stannis (which doesn't get there) proclaiming him the Rightful Ruler endorsed by himself. It is easily possible for the Starks and Lannisters not to end up in war though, if for example, Ned Stark takes the Black and his son takes over. This would leave them conflicting, but "Neutral". However, the Stark's do have major influence with House Tully (currently NPC'd) which may be persuaded to join them.

I believe during this period too, Renly just escaped with the Knight of Roses to gather up forces in Storm's End and High-garden, and is just about to marry Margery of House Tyrell. Though upon joining these factions, I assume the players there would already know this. The lesser powers do have far more flexibility though in their choices.

Things are pretty difficult though for the three most powerful factions (Lannister, Stark and Tyrell) to actually end up on the same side.