@Kolten, I just realized that you already voiced most of the core ideas in "my" theory below; I really should have read all the responses before typing this all out. I do think that you might appreciate my thoughts on intent, though. With that out of the way...
Hi all, I'm finally coming out of lurkerhood to make a post here, though I'm not sure people are still tracking this topic. In any case, here's my theory about the 5th Ideal and why it's so difficult to achieve:
In line with what we know about Skybreakers, I think the 5th Ideal involves the radiant fully becoming a vessel that matches their Spren partner's divine intent, and thus their Spren fully synchronizes with their radiant. This is why it enhances their radiant powers -- it makes them something akin to a Shard and its ideal holder, though the power is limited enough that they do not experience ascension.
Here's how I think it will go for Shallan: I think she will discover (or rediscover, I suppose) that Cryptics are actually "Truthspren," and in order to achieve the 5th Ideal she will have to be able to honestly say something to the effect of "I am true to myself and to those I love." I don't know if anyone else has already made the "Truthspren" point, but here's what I mean: We know that Cryptics are bad at lying, and that they are obsessed with both the truth and with lies. I believe that the reason they are stimulated by lies is because of how lies interact with the truth. Obfuscating the truth is alluring to them in the same way that clothing can be enticing to humans -- a bad lie is like a person wearing a burlap sack (which could be mildly alluring in an outlandish way), whereas a carefully-crafted lie is like a gorgeous gown (which greatly heightens the appeal of the wearer). As a final point, a cryptic's general appearance is a reference to the idea that truth is complex, multifaceted, and is difficult to discern in its entirety because of the near-infinite contributing factors involved. It also doesn't help that on Roshar (and elsewhere in the cosmere), peoples' perceptions shape the cognitive realm which can in turn have a large effect on the spiritual and physical -- meaning that perceptions literally alter the truth, albeit over time.
Continuing the theory, here are my thoughts on Windrunners: I think that the divine intent associated with Honorspren is not Honor, because they are not actually 100% Honor. They claim the name "Honorspren" because they are the closest of the humanoid spren to Honor's intent (something like 90% Honor, and 10% Cultivation), and because they see it as a badge of honor -- pun intended. I think this is hinted at in Syl's "two brains" monologue, and I think their true divine intent may be something like guardianship (protecting others because it is the honorable thing to do, and also because it allows them to grow). This would mean that for Kaladin to reach the 5th ideal, he would have to say "I am a guardian" and actually believe it. This would be no mean feat, as it would require him to truly overcome his belief that he is a failure. His 4th Ideal was a step in the right direction, as it allowed him to accept he cannot save everyone, but he still has a way to go before he would be willing to view himself in a completely different light.
So here are my guesses for the other radiant spren and their divine intents (based in part on the descriptions of each order):
Highspren - Divine Intent: Law, Example Oath: "I am the law" -- this one is given
Lightspren - Divine Intent: Exploration, Example Oath: "I am that which discovers the way forward" -- this one makes sense to me
Inkspren - Divine Intent: Revision, Example Oath: "I am eternal refinement" -- kinda makes sense given Jasnah's MO
Cultivationspren - Divine Intent: Memory, Example Oath: "I am an essential part of history" -- pretty weak guess, but it's all I've got
Ashspren - Divine Intent: Purification, Example Oath: "I am that which purifies"
Peakspren - Divine Intent: Resolve, Example Oath: "I have found my purpose"
Mistspren - Divine Intent: Guidance, Example Oath: "I am that which helps others follow"
I also believe that Spren like Renarin's will have a different Divine Intent based on it's connection to Odium, and thus a different 5th Ideal.
THERE YOU GO. If you managed to wade through my massive wall of text, I commend you. Any comments, contributions, or criticism would be greatly appreciated!
