Yeah, Russian follows the same logic as German, where the case system maintains throughout and across sentences and speakers. Meaning the reply is going to need the same case as the well-wish.
The reply will match the necessary cases of a prior sentence, even if it omitted them.

Удачи (тебе)!--
-- И тебе (удачи)!

Whereas if the first person used the nominative, the reply would be nominative as well,

Будь ты аккуратно!--
-- Ты тоже (будь аккуратно)!

When it comes to phrases of well wishing and "impersonal statements", it's common to lack a nominative-subject. If there is a verb in the sentence, then there is a subject in the nominative (perhaps hidden) somewhere; however if a sentence doesnt have a specific verb then a nominative noun isn't quite necessary.
"хорошего дня ты" - ты has no logical, or syntactic link in this sentence so it makes little to no sense.