Why can't I say "blivande"?
Blivande means that they’re in the process of becoming X. You could say hon är en blivande läkare/pianist/mor/ (doctor/pianist/mother-to-be).
why can't i say "framtida"?
You can, I've added it now.
"framtida" seems to be still marked as incorrect.
It's definitely accepted, so that's probably a bug then.
Why is it "för" and not "till" here?
till would mean "to", so it's in the sense that it's a questions future generations will have to tackle, rather than a question asked to future questions.
Ok. So it wouldn't be gramatically incorrect (meaning "fråga" could be used with "till" in certain contexts), but it wouldn't fit this case; is that correct?
Varför inte "Det där är?"
That is also a perfectly reasonable translation, assuming you want to emphasise the "that".