Harmony is the newest and most inexperienced Shard, with the greatest potential for growth. He could benefit from a mentor to guide him and help him grow into his role. Taravangian also mentioned planning to start a conflict on Scadrial in Wind and Truth, which gives me a hunch that he will play a role in Mistborn Era 3. With Retribution planning to involve himself with Scadrial, I’m not sure Harmony has the luxury of refusing help from Cultivation.
Also Cultivation's style isn't to be at the forefront in Shard conflicts; instead, she plants seeds to stimulate change. Conflicts themselves stimulate change. Someone also mentioned that Preservation's intent could clash with Cultivation's intent, but that’s not the Shard intent of Harmony. Additionally, Cultivation and Honor didn’t clash, so it's not inevitable that Shard intents would always conflict.
So all in all, especially with a common enemy, if Retribution does appear on Scadrial, it wouldn't be unlikely that Cultivation would appear on Scadrial. I don't like the chances of Harmony going one-on-one against Retribution.