Both of them missing the same arm, both of them lost it to a serpent of some kind, both of them have estranged daughters although Mashima was WAY ahead of Oda on that by MANY years so its truly ironic and full circle right now.
Also both top tiers in their world, enigmatic characters who are charming and well loved especially by the MCs and are like a father figure to them too.
I think Mashima was Odas assistant before or something which would help explain it? Lmao.