Regarding Rev 1:1: for SHMAINW, BDAG has
(1) to make known, report, communicate
(2) to intimate someth[ing] respecting the future, indicate, suggest, intimate
(3) to provide an explanation for someth[ing] that is enigmatic, mean, signify.
Rev 1:1 is categorized under (1) in this lexicon.
On the other hand, _The New Linguistic and Exegetical Key to the Greek New Testament_ explains that ESHMANEN (the aor indicative active of SHMAINW) can mean "to signify." Moreover, we read that "The word strictly means to show by some sort of sign, and it is esp[ecially] used of any intimation given by the gods to men, esp[ecially] concerning the fut[ure]" (page 610).
David Aune provides substantial textual evidence that suggests the NWT (New World Translation] is most certainly on the right track in its handling of ESHMANEN at Rev 1:1. After reviewing how the Greek verb SHMAINW is employed in extra-biblical literature and the NT, he writes:
"In Rev 1:1, SHMAINEIN cannot mean 'to indicate clearly.' By using the term SHMAINEIN, the author expresses the difficulty in understanding the revelation narrated in the text that follows, and perhaps even emphasizes the necessity of informed interpretation" (Word Biblical Commentary on Revelation 52A:15).
One pivotal classic text in this regard is Plutarch's De Pyth orac which reads:
OUTE LEGEI, OUTE KRUPTEI, ALLA SHMAINEI
"[Apollo] neither declares, nor conceals but signifies."
"This [text' refers to the fact that the Delphic oracle gave ambiguous advice using images and riddles and that such advice required interpretation (see Kahn, Heraclitus, 121-23)."
Quoted from Aune, ibid.