It Has to be something that happened at the reverie so Pluton would make sense, but does BB have a poneglyph reader?
Blackbeard enthusiastically researches world history and may have knowledge to read poneglyhs somehow.
I'm worried he may target Robin for this reason but he could for Pudding potentially too ofc. 3 eyed girl for the 3 themed man.
I've suspected Burgess may have a 3rd eye under his mask and thus why BB recruited him and keeps him around but the Kozukis can read and enscribe poneglyphs right? It originates with them? So Momo or Hiyori could be targeted too? Blackbeard will have known Oden personally ofc, maybe even learnt from him directly?
Shiryu is probably from Wano too and may lead into this too, especially with gis invisibility DF making potential infiltrations and kidnappings much easier. Especially if he can turn others invisible when he touched them too. Awakening maybe?