If ‘what are you doing?’ generalises, I’d say people would end up answering just the same way people answer to ‘how are you?’ or ‘how are you doing?‘.
In fact, in Spanish ‘¿qué haces?’ or in Greek ‘Τι κάνεις;’ (both literally meaning ‘what are you doing?‘) can be used, depending on formality and closeness, as greetings, and the usual answers are as shallow as ‘fine’.
In other languages, ‘where are you going?’ is a customary greeting and again it’s not expected to be answered with an honest description of where you’re physically going to, but rather with another more or less fixed expression similar to ‘fine’.
If ‘what are you doing?’ generalises, I’d say people would end up answering just the same way people answer to ‘how are you?’ or ‘how are you doing?‘. In fact, in Spanish ‘¿qué haces?’ or in Greek ‘Τι κάνεις;’ (both literally meaning ‘what are you doing?‘) can be used, depending on formality and closeness, as greetings, and the usual answers are as shallow as ‘fine’. In other languages, ‘where are you going?’ is a customary greeting and again it’s not expected to be answered with an honest description of where you’re physically going to, but rather with another more or less fixed expression similar to ‘fine’.