The power granted by the Hallucigenia
It's been made pretty clear that the titans working the way they do, the 13 year curse and only the royal family being the only ones capable of using the Founder's ability are all arbitrary limitations unconsciously set up by Ymir herself. This begs the question: What kind of power would a rational, mature and well-adjusted individual wield if they came in contact with the Hallucigenia?
It seems to me that you basically become a biological god with the Hallu power. You become immortal, with a healing factor, and even if you do die, you just incarnate in an alternate dimension where you could build yourself a new body. Most Importantly though, you're capable of creating and controlling a seemingly infinite amount of biological material, shaping it to your will with no limit to its structure or appearance. You're also able to completely control the body and mind of any of your descendants, even being able to grant them superhuman abilities similar to yours.
Man, imagine if a genuinely good and smart person got their hands on this power. I'm of the belief that absolute power corrupts absolutely and that no one person is capable of wielding so much power; but still, just imagine all the good that could be done with it if the holder of the titan power wasn't a traumatized 9 year old that didn't let themselves die and become a mindless slave just 13 years after getting that power.