My mom thinks my dad is a narcissist (they're divorced), and I can see that I take after my dad in many ways...so, here goes...
I think the type of significant other a narcissist needs is someone who highly values the things that the narcissist does well or comes naturally to them. This is because the narcissist, above all, wants to be wanted.
The maximum want you can get from someone is a function of how much they value you.
You are a collection of traits, skills, talents, knowledge, personalities, etc.
I assume narcissists wish to reach the stage of maximal "wantedness" by the path of least resistance. There's an inherent efficiency to being wanted for how you naturally are--this requires the least amount of deviation from equilibria.
I suppose this applies generally to everyone. But I think it has particular application to narcissists. Not that it'd work in every case, of course, but I think on average narcissists would have better relationships if they sought out partners with this in mind. They will most value the person who values them most.
Am I completely out in left field on this one?