If that happens it will be in the final war at this point. Everybody is still waiting for the freaking Lucci, Enies lobby arc (around chapter 400), to actually do something on panel. And he is confirmed to come back one way or another. So if Kata will ever do something again on panel that will be in the final war. Then at best I can see him by that point be one of the many helpers who will show up, so current Luffy lvl at best. He will still somewhat be relevant and keep the resemblance of some kind or rivalry with Luffy (a bit like Croco and Luffy in Marineford) yet for sure weaker even because it would be stupid to make him jump from YC1 to yonko in the span of a few weeks/months (even if I can totally see Oda doing it and don't care about logic).