Even though Unalaq merged with Various, he had not mastered the the other elements, he was only a water bender. Batu has the ability to hold the other elements for Unalaq until he can master them, however he was never given them. Favs was given all 4 elements by the lion turtles. Though Unalaq and Vary can be reincarnated he would once again have to reincarnate as a water bender. In order for Unalaq to reincarnate as an earthbender or into any other nation his human spirit would have to have posses and mastered all 4 elements given to him by Batu and then Batu would have had to have merged with his human spirit after it had posses and mastered all 4 elements. You can observe an example of this in the story of Wan, the first Avatar and Rava in the legend of Korra. When Wan mastered fire, Rava passed through Wan's body giving him the next element in the cycle, this continued until he had mastered all 4 elements. Unalaq merged with Vatu only having possed 1 element - water, which he had mastered. Vatu did not even posses any of the 4 elements as they were never given to him so it is not possible for Unalaq to reincarnate into the other nations. Unalaq"s advantage is that he has a great understanding and connection to the spirits. He can use his water bending to influence the spirits. He would be reincarnated into the water nation with a memory a connection to his past self and past wisdom so he could once again develop that ability and wisdom which would help him prepare for his battle against the avatar. It is possible in his next incarnation once he remembers and obtains his past abilities he may use them on his journey while in search of the lion turtles. The lion turtles did say to Wan that they would no longer give humans the elements and we're on Wan and Ravas side when they said they wanted to defeat Vatu. The lion turtles agreed to give Wan and Rava the 4 elements, as they wanted them to defeat Vatu and restore balance. Giving Wan and Rava the 4 elements was their best chance at defeating Vatu. It is unlikely that if Unalaq found the lion turtles they would give him the 4 elements as the believe in maintaining balance and peace. They would also sense Unalaq was possed by Vatu. Lion turtles also have the ability to take away bending. In order for Unalaq to obtain all 4 elements from the lion turtles he would have to trick them. Lion turtles can also sense how many elements a person is carrying. Firstly Unalaq would have to hide Vatus spirit, so the avatar and lion turtles can't detect it. He would also need to hide that he carries an element as the lion turtles will not let a human carry more than one because without the help of Rava or Vatu, it would destroy a normal human. The energy or Rava and Vatu gives a normal human the ability to carry more than one element, even with their help it does not guarantee the human will survive the transition. It depends on the human. They need have a wise and strong spirit. Already being fused with Vatu as a water bender may make it more difficult to obtain the 4 elements as Vatu is supposed to be seperate and carry the elements until the human spirit has mastered them lastly fusing with him. This means Vatu may have to seperate from Unalaq temporarily. The advantage to that is that the avatar and the lion turtles will not sense Vatu, just a human water bender. Unalaq will still have to somehow obtain the other elements and give them to Vatu to hold until he has mastered them. He may be able to trick the avatar asking him or her to talk to the lion turtles on his behalf convincing them to give him an element other than water. The best chance of getting the avatar to do this is if unalaq temporarily gets rid of his water bending which he can do by giving it to Vatu. How ever he can only ask the avatar once as he will become suspicious even if Unalaq changes his face and asks the same question to obtain the other elements. He would be more successful if he approached the avatar making him sympathetic that he is not a bender and ask the avatar to talk to the lion turtles asking if it is possible for non benders to directly approach them and be given a bending element, that way the avatar would save time as people would not have to keep asking him and giving non benders the ability to bend will give them a greater understanding of nature and respect for the spirits. Korra did mention her desire to no longer be the bridge between worlds but still maintain balance and peace in the world. Once the avatar has talked to the lion turtles Unalaq could approach each lion turtle giving each element to Vatu after each visit until Vatu holds all 4 then Unalaq can start learning to master them. There may be one other way. In the legend of Korra after the portals between the spirit and material world were opened, non benders started getting bending abilities. This did not seem to come from the lion turtles, so there may be a second source where elements can be obtained. People only received airbending, there were no other non benders who received earth, fire or water. This may have been nature or the spirit world's way of restoring balance of the 4 elements. If the element of a nation is significantly lacking in numbers, the spirit world creates more benders of that element in order to restore balance in nature. Unalaq may have to go on mission to find the source of the elements, he may still have to seperate from Vatu, as the spirits who are influenced by Rava will detect Vatu and warn the Avatar. Unalaq will have to temporarily seperate from Vatu but can keep his water bending which he can use for his journey into the spirit world. The avatar will definately sense the incarnation of Vatu but not Unalaq. Upon Unalaq's birth someone may have to hide him so the avatar and spirits don't find him. A spiritual sympathisers awaiting Vatu's who can detect him will have to hide him until he has grown. He may belong to a group who want to plan a rebellion?