I've said it before, future opponents will have different advantages to Kaido. It's not about linear progression in strength, it's about what they do best. Id say Kaido's best trait is his defense and tenacity more than anything. Let's say these 5 opponents are left for Luffy (just what I think, not saying it's happening):
- Kizaru
- Blackbeard
- Akainu
- Im
- Shanks
1. Kizaru is arguably the fastest person on the planet. His DF is so hax that it requires you to have enough foresight to even be able to react. Rayleigh likely mastered FS to be able to react to him. Luffy is not really at this level yet with FS. So in a way, Kizaru can be a very difficult fight for him. However, its more likely Kizaru will go down faster than Kaido would
2. Blackbeard will have likely 3 (?) Of the world's most broken/damaging fruits in combination with each other. Anyone can see why he can be more dangerous than Kaido. His logia can full negate powers (i.e. potentially render awakening and Gear powers useless) and his paramecia has extremely high damaging effects. If he has a zoan, it'll probably also be broken
3. Akainu may very well have the highest AP in the series. One wrong move from him can kill you literally. We've seen what happens when you fuck up. In an instance like last chapter, if Luffy fucked up, he wouldn't have a head (probably). That being said, not nearly as tanky as Kaido, not as fast as Kizaru. Not as "varied" as Blackbeard
4. Im is the only odd one because we know nothing about them. "If" they are strong, I'd assume they are reasonably broken and a detriment to Luffy, Comparable le to the others.
5. I think most people agree that if Shanks were to be "broken" in something, it would be Haki. Maybe he is the "peak" Haki user. But again, not the highest speed, AP, tankiness, etc in the series, but also a detriment to Luffy.
There is no particular order here, but it's clear that whoever is last will be Luffy's most difficult fight by virtue of it being purely difficult. Think of this like going Crocodile to Enel go Lucci. All are broken in their own ways, especially Enel, yet Luffy had the hardest time with a Zoan, not a Logia.