@Greg:
Currently I'm of the belief that Im has been alive this entire time and might even be Uranus; that Im is what the Elders are so afraid of.
Why is Im hiding its existence? Perhaps because it fears the return of whatever threatened its power/influence long ago.
First time we see IMU (going with Steven's translation, capitalization by me, I held shift for THREE letters by myself!) was on Doffy's lattest power rant, relating to a secret on Marijoa. We were blinded by the strawhat so we didn't see the lanky lady sulking around.
I'm going with female for IMU, the shape of the eyelids, I haven't find a male with eyeholes like that, Oda goes unisex for round eyes, flat top squared corners for males (or sharp corners, the important part is the flat middles), and rounded tops but sharp more symetrical sides for women, Caesar being the main exception and even him is questionable because how high his outer part of the eyes are.
Second, there is the implication of the national treasure being an ancient weapon, with Pluton being burried somewhere accessible, as if it was Raftel then poor Crocodile did all his scheming to circumvent the world order for naught. Shirahoshi being Poseidon, the three sides needing an ancient weapon for the final war, and the implication that there is one in Raftel.
I've implied this before, but I'll do it again, how could the WG defeat the three ancient weapons? if they were theirs, then how did they lost them? (Current implications is that ancient Nefertari escaped with Pluton, Joy Boy liberated ancient Poseidon princess, and IMURANUS stayed there), but if they were theirs, how did they lose not only the weapons but the technology to build them as well? At the very least Pluton is replicable.
Back when we had Pluton and Poseidon, the next likelly weapon was Zeus/Jupiter, then it was Uranus, and Zeus was a doppey cloud (but Neptune is a doppey king so..). The point being, there is room for a Chronos weapon.
Third: Doffy's plans.
The power is about to expire.
He needs to be inmortal to claim the national treasure (and I doubt it is just plain ol' combat power)
Finally:
"The power to destroy the world", until IMU is not referred as Uranus or that descriptor, I'm not on the she's one of the ancient weapons camp. But she's ancient and powerful, and revealing her existence threatens the balance of the world.
–- Update From New Post Merge ---
There are quite a few women with what I described as "male eyes" as in the squarer form, notably Madam Shirley, Smoothie and Praline. So I might be wrong, but a lot of characters follow this rule.