Your guess is as good as mine here, but Marvel is asking “Who is Voyager” and teasing that her identity will be more fully explained on September 29th.
Connected to the Legacy titles, perhaps she plays a role in enabling characters to be linked across the titles? There is at least one existing character called Voyager in the Marvel Universe who comes from a place of interconnecting dimensions. That sounds promising. Maybe she’s a version of that character?
Or something completely different…
The timing seems related to the release of the first issue of Marvel Legacy on September 27th, too.