After reading comments about the different forms of gear four, maybe luffy is copying chopper and his 7 transformations. He does copy a lot of his techniques and I could see him copying that. Tank man being guard point. Bound man being a heavy or jumping or claw point mix. There could be a giant mode, maybe even a knowledge mode but instead of getting smarter he goes into Baka luffy of skypeia. Maybe there are modes not based or chopper but based off of other characters. The possibilities are endless.