Now, in some languages similar things genuinely appear in some clauses, and I wouldn't be surprised if such a thing even appears in all clauses in some language out there. One common such 'main clause complementizer' is the question marker.
Here appears a thing I've seldom seen conlangers do: force complementizers to appear in certain situations with main clauses, but not in others. Maybe negative clauses require a complementizer, maybe certain kinds of statements require them.
As for 'certain kinds of statements', in Swedish, 'att' (similar to English 'that' as a complementizer) sometimes introduces a clause (whose word order then is like that of subclauses), without any main clause, where the statement expresses disdain, admiration or agreement for a fact thus stated:
att han törs!(that) he dares!Using complementizers occasionally or regularly for main clauses can be an interesting way of enriching one's syntax as well. One hypothesis regarding verb fronting as a way of marking questions is that the verb actually moves to the zero morpheme question complementizer, and thus is a sort of realization of that complementizer. This of course changes details in the word order. One could have the language sometimes force the subject or object into the C position, and this would change other word order details. Maybe moving the subj to the C position breaks reflexive binding? Maybe it breaks verb congruence? Maybe moving the object breaks transitivity, making a transitive subject marked absolutive (if the language is ergative).
how dare he?
att hon gör!(that) she does!
she sure does!
Of course, the presence of an explicit, non-zero complementizer could, as in Swedish, force subclause word order, if there is a difference between these in the language. Thus maybe all negative clauses have subclause word order?