What are possible translations of τί δε? So what? what’s next? So? Then?
In many cases, perhaps most, simple “What?" may make the best translation.
A couple of examples from Aristophanes’ Frogs, taken more or less at random:
Dionysus: δέομαι ποιητοῦ δεξιοῦ. οἱ μὲν γὰρ οὐκέτ’ εἰσίν, οἱ δ’ ὄντες κακοί.
Heracles: τί δ’; οὐκ Ἰοφῶν ζῇ; (“What? Isn’t Iophon alive?”)
Slave: … καὶ γὰρ ταλάντῳ μουσικὴ σταθμήσεται.
Xanthias: τί δέ; μειαγωγήσουσι τὴν τραγῳδίαν; (“What? They’re going to weigh tragedy?”)
Of course, there’s no one-size-fits-all translation. Context will vary, and much will depend on intonation. And τί δέ is different from τί δαί, τί δή, and plain τί.