The biggest problem with Shadowland is that while it works with the Broadway production (where everyone already saw the movie), it's a big spoiler alert if it was in The Lion King. As great of a song it is, to see and know that Nala left the Pride Lands in search of help would mean that when she discovers Simba in the jungle, the audience is already expecting it - after all, they know Simba is alive, and they just saw Nala run off. The surprise and shock (Que Timon's jaw dropping) that we felt when we first learn that the lioness who wanted to eat Pumbaa was in fact Nala would not have bee there if The Lion King had Shadowlands in it.
So while I do love Shadowlands, it has no place in the actual movie. It is best left in the Broadway production where it is used.