That's a pretty tough one.
- Moria can try taking Ace's shadow, reforming the shadow be a whole shape, then sticking in back in Ace so his body is complete like the Strawhats that got burned by sunlight in Thriller Bark. With Hogback's help they can just bring him back as a zombie, but that wouldn't really be Ace and the personality would depend on the shadow.
- Sugar can combine Ace with some fodder to create a Headcracker soldier. You can blow holes in their stomach, you can cut off their heads, they have some incredible resilience. Maybe a regular toy soldier would survive this damage too. Ace is alive, but nobody knows who he is.
- Newshi combines Ace with the decuplets. Only one of 11 is has hole in their stomach, so I can't imagine their combined form would be on the verge of death. It would be an awful restriction of the Gocha Gocha if Newshi could only use it with his twin brothers and sisters, those are probably the only people trained to fight in combination with him.
- Mont D'or traps Ace in a book prison. Big Mom said "The animals trapped in books will never age. What's in a book should never change". It's possible Ace could be kept permanently on the verge of death inside a book. How cruel.
- Bello Betty's Pump Pump gives Ace's body unbelievable strength to fight to live. He survives long enough to get an organ transplant or mechanical body.
- Ivankov does the same as above with hormones.
- Wapol eats Ace along with some junk and other people, out comes a completely new creation with Ace's head in a new body.
- Big Mom and Pudding take Ace's soul and put it a homie. The Memo Memo gives the homie memories of its old life. You effectively have Ace again for however many years he was supposed to live naturally.