It's not that fleshed out or deep tbh lol
Tho my main thoughts are summarized here
I do think blackbeard getting a third devil fruit is definitely a big possibility tho nonetheless.
If i were to bullshit it to fit into my head canon maybe
Blackbeard
- Darkness fruit represents xebec
- Quake fruit represents newgate
- Possible 3rf fruit is unique to him
As for shanks its even harder to guess because their haki is even more mysterious but
- A specific form of haki ability/skill xebec specialized in
- A specific form of haki ability/skill roger specialized in
- A specific form of haki ability/skill shanks himself specializes in