So I know there was a blog post talking about how doing "what ifs" is not appropriate, but if this gets deleted I don't care. Plus, I think this topic is a way to change the entire story line in an interesting way. Also friendly reminder of SPOILERS!
I found an old blog post which touched on Hawkfrost, it mentioned the idea of the dark forest actually taking over the lake. Since it was a measly sentence, it wasn't noticed much, but got me thinking. In order to imagine a what if scenario, we need to set a set of circumstances. I think I'll go along with Hawkfrost, Tigerstar, and Brambleclaw original plan. The plan to kill the leaders at a gathering and then claim themselves as leaders. Hawkfrost taking over RiverClan and ShadowClan while Brambleclaw takes control of ThunderClan and WindClan. So I don't know how Brambleclaw would do that since we know he wouldn't do that, but let's say the plan works and is foolproof, except for one thing: the fact that the clans would not just obey. You've got a lot of highly trained warriors here and they're not going to bow down and show respect to the cats who have just killed their leaders. Personally, it would be interesting if that happened, but Hawkfrost is killed by Brambleclaw before they actually are going to take action.
But we'll just pretend that the plan was actually good, they don't kill the leaders, but simply "open their eyes" to the fact of how much stronger the clans would be if they united. Hawkfrost is a good persuader, he could explain it somewhat like this. "Bad winter, no prey? That's not an issue with all of us together. Or a fire, you could just stay here. Or a drought, after all, our superior clan has a river. We wouldn't have to fight anymore, no more border issues. Foxes and badgers wouldn't stand a chance." It really sounds nice, but there's always a catch. You know the whole, we hate half clan blood or kittypet blood and all that racism. So let's say, Hawkfrost convinces the clans, and he has multiple followers. There is a lot of blood spilled and eventually the cats against it are forced into the new clan. This is where the story takes a turn and our cat heroines are forced to bit their lip (if cats do that) and allow themselves to sit throw it. They formulate a plan how to bring the four clans back and escape from this communist-like clan, firstly they must gather support, escape, make another clan and convince or fight the others back into four clans.