Roger is a swordman, Oda said he wants Zoro to be the WSS EVER which means the WSS Zoro would be stronger than Roger, and for that to happen he needs his ultimate goal to be also the WSS EVER or it doesn't make sense for Zoro to achieve that.
A lot of things point at Mihawk being the WSS Ever through this statement and having the strongest blade in the living history; the only Black Blade that has been achieved through a Saijo Grade Blade Yoru.
Only Mihawk has a statement like this from Oda :