While I am able to definitively answer the first two FAQs, this is the difficult one. Since we know nothing about the next Persona game, it is impossible to give a complete answer to this question. In fact, Persona 6 has not even been officially confirmed, though it is likely going to come out soon enough after the success of 5. When it likely does release someday, the question of whether or not the Phantom Thieves will be in it is an interesting one. Thus far, the series has had an interesting relationship with its past games. Despite being numbered sequels, they are all essentially standalone titles, with the exception of the Persona 2 duology. That said, there are almost endless references and even cameos from past games in each title. But not playing that previous game does not take away at all from these moments since they are so small and insignificant at times. In that way, it seems inevitable that the Phantom Thieves will be referenced in some way at least in Persona 6. After all, Persona 5 is the biggest game in the series so ignoring it would be a shame and I doubt Atlus would do that. However, this could just come in the form of a news channel segment with a particular character or a poster of Ann as a supermodel, for instance. Those are the standard references that we get in Persona games. That said, I would like to see a much more significant Phantom Thieves appearance in Persona 6, personally. We have seen party members from past games return as playable characters before but only in the case of Persona 1 and 2. That has not happened since and I think it is high time for that to change. I am absolutely biased but I would love to see either Akechi or Kasumi (or both) return in Persona 6 as playable characters in the party. Even with Royal, they still had far less time than everyone else while being the best two Phantom Thieves members around. We could even see a Persona 2 situation where depending on the gender of your character or another choice, it determines which one joins the party.