If I had to place them on a tier list I already have Zoro just above Law right now. Forget growth.
The only issue here is this. Law is a hax based fighter. Hax based fighters do not follow traditional scaling. They can virtually beat anyone under the right circumstances. Oda can make Law as strong as he wants thanks to this.
Ope Ope is so broken that the only thing holding it back is Law. If Law's haki, stamina etc. Improve, good luck convincing anyone that he can't defeat whoever he's matched up with given his df. Law's ceiling is the Ope Ope's ceiling. Law is the limiting factor of his df. More stamina, better base stats, better haki = more Ope Ope shenanigans.
I've said this before but Law's df is more suited for someone like Imu or another endgame villain. If Law becomes a solid top tier he has a shot at defeating any single person in this verse due to hax. Law can virtually do anything conveniently when the time calls for it. Look at how he pulled R Room out of his ass, the room doesn't actually have any real limits or logic to it, so it's very easy for Oda to have Law keep up with anyone in this verse.
The Ope Ope is the most convenient fruit for Oda to work with.
Law's fruit doesn't seem to be
that crazy. His hax is partially limited by strong Haki - that is, not armament in particular. It is just that overall Haki, as in spiritual power/ambition can simply shut down the Ope's abilities entirely. Seen with Law being unable to move Kaido and Big Mom, with Law being unable to amplify that Hax with Haki either to cancel the opponents Haki out. Pure hax, e.g. moving people, taking their heart or cutting them into pieces probably doesn't work well against top tier Haki users. He has to physically harm them. While Shock Wille and Puncture Will are built upon hax (by bypassing Big Mom's durability with K.Room), those abilities still work by physically damaging opponents from the inside out with shockwaves. And while being strong, Big Mom wasn't done even from several blows, although admitting the Ope fruit is strong.
Compare it to the admirals logias. Aokiji could, with a free shot, freeze Big Mom to the core and shatter her. Akainu could blow holes into her and melt her entire organs. Not only extremely lethal, but also destructive, with large AoE and awakening on top to terraform environments and turn them into waste land and permanently altered weather. Or take Enel, who can bust islands with one blow, rain lightning bolts down across the country, launch precisely aimed attacks from miles away and amplify his CoO, while also being extremely fast and lethal (knocking everyone out with 30 million volts, which is just 15% of his base strength, not even considering a possible awakening, which could replicate the thick thunderclouds he needed the Maxim for - free island level nukes).