Keith and Lachlan working together to save Ewen - OMG! I don't have a strong impulse towards fic for FotH, the book's one of my favourite books ever but I don't really feel that fic urge about it, but for Keith & Lachlan as two points of a triangle with Ewen, I could definitely be persuaded otherwise... thinking about it, that's how you save Keith from his fate at the end of the book, first you have to save Lachlan. Because that's the fundamental tragedy: he worships Ewen, but Ewen takes him for granted. I mean, there's a hierarchical chain of loyalty, Lachlan -> Ewen -> Lochiel, but we see Lochiel reciprocating that affection towards Ewen and looking after him in return in a way that we don't really see Ewen doing for Lachlan.

In other words, yes, I like this idea a lot!