It will be related to Zoro training under Mihawk. If lucky, we may see some great Perona-Mihawk-Zoro interactions during the 2Y.
Who knows! May be we'll see Shanks. Zoro is still the only one who hasn't seen any Yonkou (except BB, but he wasn't a Yonkou then).
But I don't believe we will get childhood/origin flashbacks