FOOD FOR THOUGHT: Let's get a meaty discussion going. 
But not a subject complement, right? Because those are nominal, and which leads to this question: why can't
next be a preposition here,
Who is next? (aside from what's obvious: it doesn't have an object, but particle prepositions can occur without an object in phrasal verbs, right? They're often called adverbial in that position, though, to account for it.
I believe that in phrasal verbs, the entire construction is a verb, and not split into verb+adverb+preposition, as in "stand up for" that is treated as one grammatical unit)
Also, a phrasal verb may be interposed by a noun or pronoun, as in "make out": He made himself out to be the boss.
That is, non-linking forms of the verb BE take adverbs in the predicate, but the form and function of those adverbs aren't always the same. For example,
She's in the house.
Form: prepositional phrase <nominal>
Function: adverbial <tells us where>
So, given that adverbs give off this kind of duality, why couldn't
next in
Who's next? be a preposition in form, that is, be nominal and thereby privy to the category "subject complement"? Why is it called a predicate adverb?
In short, why does the verb BE have to be split into linking and non-linking kinds?
What's the difference in meaning here?
Adverb: The person is
next => Adjective: the
next person
Adjective: the
next person => Adverb: The person is
next.