The portal should have been closed. It was a mistake that those two worlds came to be in the first place. Even before Wan's time the Spirit World poured into the Actual World, because of the Vaatu and Raav's constant battle, thus creating the Spirit Wilds, rendering humanity to live under the protection of the Lion Turtles, causing them to be isolated. Its going to happen again, but unlike lats time humanity had time to refine bending arts and technology, its going to be a war between worlds.
Plus the threat of the Spirit World being exploited is inevitable if a convenient door to it exists. Come on lets be realistic since when in humanity's history have they ever been responisble? A rich world next door, business people are only going to care about monetary value not it sacred value. The Spirit World is suppose to be a spiritual place where a few truly enlightened and spirtual openmind could reach, The two spirit portals are located at the opposite ends of the Earth, in the most harshest territory. The journey reaching them is suppose to be both a physical challenge and a spiritual challenge.
Having one right next door in a metropiltan city, cheapens its.