Hancock and Luffy's kid for sure to have CoC can't be diluting that gene pool with Nami trash ass DNA.
Hancock and Luffy getting together would be poetic in the aspect Hancock trusts no other man.
Hancock as well is powerful enough to stand side by side with Luffy unlike Nami who has to hide away at the back

not to mention Luffy and Nami are like brother and sister