can't really do a referendum 10 years later, after invasion has already happened, after occupation/war has pushed a substantial amount of the population into exile, and after scores of russian forces have flooded the territory.
for it to have any appearance of legitimacy, a referendum would have to happen before changes take place, and it would have to be carried out by a neutral power that is considered trustworthy by the major world powers but also has absolutely no ties to either side of the issue.
now, where the hell would one find a power that is
>able and willing to carry out such a referendum on foreign territory
>has no ties to any ukraine-aligned country (this rules out ukraine, america, all of europe, japan, south korea, etc)
>has no ties to any russia-aligned country (this rules out russia, belarus, china, iran, north korea, etc)
seems impossible to me.