Erieblogging: Day Twenty-Nine

My Erie question for the day is still about relatively unguided Erie choices, that is, cases in which  a procedural matter faced by a federal court sitting in diversity (or supplemental jurisdiction - or bankruptcy? or CAFA? or statutory interpleader?) is not covered by federal enacted law (such as a federal statute or a Fed. R. Civ. P.). My question is this: What is the grand unified theory?

A federal court facing such a case is supposed to balance the twin aims (whatever those are), which recommend borrowing the rules that would be used by a forum state court, against countervailing federal interests (whatever those are), which argue in favor of a uniform federal common law rule - while taking into account states' interests in their rules applying in federal court (which states? what types of interests? how can we tell if they exist?). Today's question is what happens when you put all of these in the hopper. How is a decision produced? What is the balancing theory (if balancing is what is going on)? What is the grand unified theory?

Two more days. Tomorrow: Erie in DC. The final day: What we can learn about Erie from the courts of other nations with federal systems.

(Parallel posted on Michael Green's Civ Pro Blog.)