why would crocus respect and be friendly with someone like shiki ? can you see zoro doing the same with someone like blackbeard or even doflamingo ?
pretty sure shiki tried to kill roger so I wouldn't compare their relationship with kid and luffy
Black beard- yes he seems respectable
Doffy- is an unhinged sociopath so probably not
Shiki never once tried to kill Roger in fact he wanted to ally with Roger they only fought because Roger refused to team up with him which lead to the edd war battle
And the only canon meeting between them we know about is God's Valley where Roger fought Rocks and likely didn't fight shiki in that battle
Shiki was even shown visiting whitebeard in chapter zero
As for that 20 year plan- Shiki escaped impel down 24 years ago he's kinda late and we saw a version of that plan get thwarted in strong world no reason to see it again
Tbh neither Gaban nor Shiki offer much we don't already know I just prefer it to be shiki of the two cause I think he's the better of the two characters