Can a-nis work in this sentence in place of an-dràsta?
Why is the word ann not used.
There is no need for it - Tha i blàth = it is hot (tha + pronoun + adjective). You would use ann with a noun to say "there is x" (tha + noun + ann).
If I am understanding you correctly "ann" is for saying something exists and the structure used in this exercise is for describing that something, right?