In modern-day Shush, (if any Jews lived there), what day would Purim be celebrated on?
Ancient Susa had a wall since before the time of Yehoshua (according to Wikipedia), so that's not a problem.
The real question is, i suppose, is the modern city close enough to the ancient one to be considered the same?
I do know that in T'verya, even though the ancient city had a wall, the modern city (for the most part) does not celebrate Shushan Purim, because it isn't really so close. As for those who do, Rabbi Daniel Sperber says that it is a minhag ta'ut, a mistaken custom.