Oda pretty much told us she losing.
yea in an event ( interview ? ) some time ago Oda acted like her , Vivi , and Sabo were in bad situations iirc
plus , Coby is aiming to be an Admiral , probably will be the strongest Marine ever even . and will have a sparring with Luffy in the epilogue
first major accomplishment Oda gave him is being the Hero of the Rocky Port incident , in which Law was involved .
second major accomplishment , defeating Hancock ? will be quite good for him .
plus i think he could be an opponent to Brook or Usopp , or the Tenth Strawhat in the Final War , post defeating Blackbeard Pirates who will be stronger than a standard Emperor crew like Beasts Pirates .
these are my headcanons , but i think the matchups between SHs and Marines are going to be like this :
• Luffy vs Akainu ( Sabo is also possible , but anyway , continue to other matchups )
• Zoro vs Kizaru
• Sanji + Jinbe vs Ryokugyu
• Franky vs Chaton
• Robin vs Momousagi
• Nami vs Tsuru
• Chopper vs John Giant
Tenth Strawhat ( ? ) , Brook , Usopp vs who ? i couldn't figure this out , but the strongest candidates are X-Drake , Coby , Smoker