Well, some might have a plot where the OC is a healer and has to tend to Kenpachi's wounds after a particularly brutal battle. This could lead to a series of interactions where they start to understand each other on a deeper level. Another idea is that the OC is from a different realm and is brought to the Soul Society, and Kenpachi is assigned to keep an eye on them. Their personalities clash at first but then they find common ground. There could also be a plot where the OC is a mystery to everyone, including Kenpachi, and he's determined to figure out what their deal is.
A common plot idea is the OC being a powerful but unrecognized fighter in the Soul Society. Kenpachi, always on the lookout for strong opponents, discovers the OC during a random encounter. This encounter could spark his interest, and he might start to pursue the OC for a fight. As they interact more, the OC could also teach Kenpachi something new about combat or life in general, leading to a unique relationship. Also, the OC could be part of a prophecy that Kenpachi is somehow involved in, which would create a complex and interesting story.