Here's my idea:

After Rainstar and Hawkstar retire, Lilystar goes to SunClan trying to make peace. Duskstar hears her out and realizes that DawnClan was not responsible for Coldheart's death and agrees to a truce between the two clans. One of them or one of their deputies brings this up in the next gathering, implying that Coldheart was killed by NightClan. NightClan is outraged and MoonClan comes to their defense. DawnClan and SunClan make an alliance hoping to protect themselves as tension grows. Eventually something happens between SunClan/DawnClan and MoonClan/NightClan, starting a battle between the two.

This could bring NightClan and MoonClan into the plot and start the alliance between the two that we thought would be really cool before, plus it would force SunClan and DawnClan to be a major part of the plot, not merely backup for NightClan and MoonClan.  What do you guys think of it?  Have any ideas of your own?