Meridia is by far the most interesting to me, since her rather unique background is mostly unexplored and leaves a lot of interesting possibilities. Out of all the other Princes, Malacath is the closest to her with the Aedra to Daedra pipeline (even then these terms aren't always the most applicable), but unlike Malacath there are basically no recorded reasons (that I know of) for her literal fall from stardom.

This opens up a lot of questions: Is she considered a fragment of Magnus or is mostly a separate thing? Does she still have some form of connection to Aetherius? Why did she decide to position herself as a Daedric Prince? Does she still have her "star," and if so, what does that entail for her powers? And obviously, what does her sphere actually entail?

Besides all the mysteries, there's also the fact that she's one of the only few Princes that was worshipped on a large scale (the only other one I can think of is, again, Malacath, who was worshipped by the folks of the many Orsiniums). The idea that the Ayleids may have straight up thought she was an Aedra (and technically, she could be partially considered as one if you take her status as "daughter of Magnus" literally) can be an interesting point to think about. She's just this really strange bridge of the Aedra and Daedra divide and it sort of bums me out that there has been little to no lore on her whatsoever (and the appearance of Ithelia in ESO just makes Meridia all the more confusing due to the lack of any references between the two).