Let me simplify your question to make it easier to analyze. Your sentences are, approximately,
1) "What can there be" + (adverbial clause)
2) "What can be" + (noun)
Sentence 2 works with an adjective, adverb, or noun (clauses)
"What can be good?" (adjective, predicate complement)
"What can be in Paris?" (adverb, modifies the verb)
"What can be the best?" (noun, predicate complement)
Sentence 1 only works with an adverb because "there be" is not a linking verb. In order to use an adjective or a noun, you need a linking verb that links the subject of the sentence to the predicate complement. Here are some other examples with linking verbs:
"What can sound good?" ("sound" is a linking verb)
"Who can become a doctor?" ("become" is a linking verb)