Brosnan touched on this topic in a recent interview, which seems like an interesting prospect.  However, I could only see a flash-forward or cameo-type scenario as a plausible Pierce Brosnan role as Bond in Bond 26.  I certainly don’t think the audience has an appetite for a full “Bond the septuagenarian” movie, regardless of how clever or dramatic of a movie Denis Villeneuve and Steven Knight could conjure up.  It’s a fun scenario for discussion, but in practice it’s not that enticing.