To be honest, Greenbull's fruit is looking really OP like hashirama in Naruto (just look at how easily and effortlessly he could control a dragon fruit user the size of kaido), but sanji is a fire user, and not a random one at all (blue fire). If you rank all the flame users in One piece world, he would be in the top 4 (sabo, akainu, sanji, King). We all know how fire counters Wood/Forest, so i'm actually really intrigued about how oda will manage to set up the fights between monster trio and admirals. Will sanji fight Greenbull because he is technically the best choice/counter to him ? or Oda will decide to chose another path where he doesn't like this idea of sanji countering him with fire, and decides to put sanji against kizaru... It would be interesting and a really great way to discover Kizaru's secrets, because honestly I still think Kizaru is really important to the storyline related to Vegapunk either it's him or not, he is surely hiding his secrets (i'm pretty sure he is smart) And a fight between zoro and kizaru wouldn't highlight all this, but kizaru vs sanji would do it easily with all the things germa has to do with marines and vegapunk...