I imagine a lot of the answers here will be along the lines of "to be a baby again".
@Frungie 's wish for acceptance is a good one, It would be nice if the world could just accept us how we are even if they have no interest in it themselves in the same way they everyone accepts different people like different foods or clothing or colours etc.
If I had one wish though and it had to be ABDL related would probably be exactly the same as if I had one wish of anything at all...
"I wish that I could, at will, instantly transform my physical body to any age, gender and physical appearance and be at peak physical health for it."
This wish works on so many levels, for a start it's technically immortality. Anything that doesn't kill you instantly can be whisked away simply by willing yourself to transform (return to peak physical health each transformation), broke or lost an arm no trouble, lungs packing up after years of smoking easy fix, been poisoned no problem, developed life threatening condition... Everything can be instantly fixed by simply becoming a day older or younger.
Secondly you can completely change your appearance so people won't notice you living for hundreds of years (though getting official new ID's could be troublesome).
Third, Gender swap any time you like, able to explore both sides of life.
Finally, able to become a child again, just become a child and live happily at any age you choose for as long or as short a time as you like and able to be an adult anytime you like to get things you need. You could even, as an adult, arrange an "adoption" with a nice family then become younger to live it out, over and over again.