I could see a them doing a OC character around the time randos in the Avatar world were getting the ability to airbend after all the spirit shenanigans. At that point we're learning Airbending from Tenzin/Korra, there's a whole new cast besides Lin/Pemma/Zhu Li/one-off-girls (Suyin, Kya, Kuvira, Ming-Hua, P'Li, older Jinora, etc). We can still deal with a bit of Korra's angst and because it's the love route we don't have to break her like the slave route. Plus, from book 2 and on, the love routes clearly have more effort and time sunk into them.