I came across this sentence:
Was gibt es denn so Spannendes, dass du mich extra morgens um acht anrufst?
This seems to be the structure similar to English: "... so adj/adv that + clause". I think its meaning can be translated as "What is that, that is so exciting that you call me at 8:00 in the morning?"
But clearly to me, Spannendes is not merely an adjective or adverb, due to postfix "es". If it were just adjective, there would be a "to be" nearby. If it were adverb, there would be a verb nearby. (here "geben" is not appropriate).
In German, we can replace "Adject + Noun" with simply "Adjective" (similar to english, e.g. "the rich", "the poor"), and capitalize that adjective.
But which noun does this adjective modify? It could be implied as "etwas" but then the whole grammar structure of the sentence is incorrect.