Zoro will get good feats against Imu himself imo. I am basing this based on possible ray's bounty which can be above Kaido's.
Eh, I'm not denying Ray's bounty can be above Kaido's but if your 5 billion bounty for Zoro theory hinges on him leaving a small scratch on Imu that never gets mentioned again after the chapter it is made (similar to the small scratch he left on your fave that never gets mentioned again after the chapter it was made), your theory then hinges on two things:
1. Lanji doing a similarly irrelevant feat to Imu, because let's face it, here, Lolda sees Zoro and Lanji as dead equals and interchangeable (even if we don't). Zoro and Lanji reached 1+ billion bounty together, so your theory relies on Zoro and Lanji also reaching 5+ billion together. I'm honestly still iffy imagining Zoro and the Cook reaching 5+ billion simply because they scratch Imu once, whereas WB and Roger had to be world-beaters to achieve 5+ billion, ngl.
2. The fact that Zoro and the Cook receive a bounty after helping Joyboy save the world...in front of the whole world. Just...feels weird asf. Maybe this is just me being stubborn, but I'm leaning toward the SHs final bounties being received pre-final fight.