I like the idea of a secluded place where people are forced to wear diapers.
Have you seen the game Perpetual Change? It's literally an ABDL on an island. (But the society there is not ABDL)
I specially like the idea of an evil organization regressing unwilling participants to become adult babies, physically and mentally.
Instead of an island, I would choose a bunker. True, it is objectively worse than an island, but it's just to conceal the experiment: Can you "delete" a person's mind? This bunker will be running that experiment, with unwilling subjects that regress over time, with drugs, hypnosis and lots of children toys to make them feel at home. Robots and AIs help the doctors run the place, but they will become part of the experiment.
After that, they will be retrained to become an unknown government's loyal solider/killing machines/super assassins/etc. However, because the nuclear apocalypse occurs out of plot convenience, they effectively got cut from the entire world. The bots start malfunctioning, and force the doctors to be regressed themselves.
It is up to you, the Diapered Deviant to help everyone escape, leave them trapped inside or help the bots complete the profound experiment on Vault 89.
Fallout DV: Coming soon...