Turkey has been floated as a serious EU candidate for decades now. Most of it is not in Europe of course, and it has a large population. Turkey refuses to address its past with the Armenians, Kurds and Greeks which is a major concern.
The other question is what its border control is like at the eastern end.
There are already large numbers of Turks in western Europe, particularly Germany, so presumably some of them would vote for it.
> There are already large numbers of Turks in western Europe, particularly Germany
Define large numbers. It's around 5% or even less in Germany which hosts the biggest Turkish community to the best of my knowledge, and way less in other European countries. Bringing Turkey in imo might dissolve the Union, I don't see anyone trying it soon.
> Turkey refuses to address its past with the Armenians, Kurds and Greeks which is a major concern.
It's also a dictatorship and its population is probably not in favor of liberal democracy - most (all?) Muslim countries aren't.