Pretty clear Oda wants to gun for actual god-tier opponents instead of someone who caps out at Roger's lvl max
Who though? as far as we know, there's no villain with God powers besides Imu. Loki is Luffy's Tsundere best friend and immune to DR so he doesn't really work, and Teach should probably last until the end game(people can debate if he goes before or after Imu, but probably around the same general time) and while people can differ on how strong current Luffy is, it's pretty clear he isn't ready to rematch Imu next arc or whatever.
Akainu is a "Op contender" and has a personal connection with Luffy by scarring him. Luffy should at least have one 1v1 fights before going to Laughtale,at least,and I don't think anyone would want Luffy vs a Gorosei.