“I will say, both those characters [Booster Gold and Blue Beetle] are characters we’ve talked about.
His response confirmed that both Team Arrow and Legends of Tomorrow could have had a different team member, and that the future might still possibly see Blue Beetle and Booster Gold appear.
So although there is no sign of Blue Beetle and Booster Gold in the Arrowverse at the moment, it’s certainly not impossible that fans will see them in future seasons on one of the shows.
Whilst it may never happen, it’s interesting to note that Guggenheim had a ready answer and that Blue Beetle and Booster Gold are clearly still on his radar.