Unless I’m badly mistaken, we only know two 5th Ideals at this point. The Lightweavers’, of course, is to speak one last truth about themselves, and the Skybreakers’ is “I am the law.” Since we haven’t actually seen a Lightweaver speak the final ideal, we don’t know if there’s a theme to what their “final truth” has to be. Taking a little leap of faith, I would think that this truth would be something simple, but powerful. In Shallan’s case, this might be along the lines of “I am enough.” (Now that I say this, I can’t imagine that Shallan’s final ideal will be anything other than this). Leading back to the Windrunners, I think their final ideal is probably something brief that embodies what their order is at heart, like the Skybreakers being dedicated to the law, or the Lightweavers being dedicated to truth. Some ideas might be “I am Honor,” “I will protect” (maybe that’s a little too close to the first ideal?), etc.