"Now what do you want?"

Translation: Co zase chceš?

[NinaOBel]

Why we must use "zase" (again), but not "teď"?

[igamenir]

I think "co chceš teď/tentokrát?" or "a teď chceš co?" are used.
"Co teď chceš" or "Teď co chceš" are NOT used. But zase is very usual figure of speech, to express irritation from repeated requests.