1. Grammaticalization. To be a discourse marker, or space builder (in the cognitive linguistics sense), the construction must be developed from an originally more meaningful one (in the lexical meaning sense) to a more grammatical (in the funtional sense) one.
2. Stability. The construction must be relatively fixed both in form and meaning in its current use, but this does not imply that no alternation of form is not allowed.
3. Discourse-pragmatic implicature. That is to say, once an interlocutor hears the construction, s/he would immediately infer what will come after that construciton, including both grammatical and discourse-pragmatic information.
Hope to hear more insightful suggestions from other researchers.
You have broached up a very interesting question. Language regarded as linguistic or pragmatic phenomenon involves a subtle distinction. If the linguistic expressions are abstracts from the speaker and the context, then what they refer to is not determined by the intentions of the speaker and/or the contextual factors. In other words, the meaning aspect denotes the literal meaning portrayed and manifested through the linguistic and semantic conventions ignoring extralinguistic influences related to actual use of the expression. While purely linguistic conventions deal with the literal meaning of an expression, the subject of pragmatics is what and how speakers communicate by using that expression. Differently stated, the conventional aspects of linguistics take care of the meaning as encoded in the lexicon, whereas pragmatics deals with the communicative elocutionary aspects of speaker meaning in concrete discourse contexts.
It will be useful for you to read the following article by Trauggot: https://www.researchgate.net/profile/Elizabeth_Traugott/publication/228691469_The_role_of_discourse_markers_in_a_theory_of_grammaticalization/links/543c0a650cf204cab1db6504/The-role-of-discourse-markers-in-a-theory-of-grammaticalization.pdf