Honestly, the Tenzen angle makes no sense. The MC obviously comes from the time when Korra or someone later is the Avatar being that we're introduced to him as a native of Republic City. Considering this whole story is about him gaining power and lust that are tied with the Avatar's legacy and that his spirit is meant to become stronger for some unknown destiny that culminates in his time, wouldn't it make more sense if Book 4 was done in his normal perspective over him having to possess another person's body? He could've been one of the fledgling Airbenders after Book 3 (in LoK) and his interactions with the cast would've been a bit more organic (and I don't mean the fucking). Plus, there would also be some closure between him and an older Korra and Toph, which I'm sure would've been a bit more heartfelt.
While the Tenzen approach (not my cup of tea, personally), isn't bad narrative wise and gives access to his family, having the character be himself would've been just as good, if not better, a choice.
Unless we're looking at what happened similarly in Book 2 where either route relates to a different character (though flipped with the OC being the Love Route and the canon character being the Slave Route possibly with their own unique characters to associate with).