If Oda wants to remain within the established ability concepts of his verse - physical make ups, haki and DFs -, I think the only way Imu could become an unstoppable force with that kinda stature is by having a surprisingly very powerful DF. Even DFs like Foxy's and Barto's barrier fruit, that would be somewhat fitting for the "physically weak, but op"-trope, aren't op to such a degree.
Imo the only ones that have such a potential are logias and in this case specifically a water logia. I mean think about it: Within the basic DF ability, the user would still start off with just non sea water- water beams and stuff. That's nothing special at all.. It's only when you add advanced abilities on to it, that the ability becomes more and more powerful, until it becomes super op. A fluid and slippery ability like it, becomes impossible to hit with FS for example.
And going by the fact that logias have access to avatar forms - Amaru and Broccoli form - this fruit might give you the means to form an avatar out of the biggest resource there is on the earth- the occean. Sounds ridiculously op, but if Luffy ever decides to go into something like ggn giant Snakeman or something along those lines, it's also not exactly within the realms of the average top tier level.
And what's even better: You can basically remodel the surface of the whole planet. If Aokiji and Akainu can create an entirely new climate zone, Imu can likely create great floods thanks to massive amounts of rain. Would explain a lot of the phenomena within the one piece world and would also tie into Alabasta a good bit. And we would also know, which fruit represents the rain god, that was named dropped in Skypia.