Japan? Their natives were few in numbers (less than 2%), so it can be considered a Homogenous nation.

That's a good example (if we ignore their attempted expansion). I guess being an island helps.

Is Japan considered an Empire (on the power/clout level, not how they nominally call their head of state) before expanding to Korea and Formosa? I don't think it is.

I think we can keep out immigrants who do not want to fit in without having to resort to genocide.

I think several millennia of European history show that they actually cannot do this without resorting to genocide. This has literally happened in Europe within my lifetime so forgive me if I ignore your unfounded assertions that 'this time' Europe will get it right.