i have a few Leona ideas kicking around my head, mainly about when she reunites with Dante + Vergil:
the earliest, she feels the rising of Temen-ni-Gru and being a curious teenager, goes off poking because it feels like her dad kind of. she manages to keep Vergil and Dante from really getting into it by throwing herself between them. idk, i haven't poked at this one much
Leona and Dante reunite between 3 +1 and she goes with him to Mallet Island and together they manage to free Vergil and then (literally) tear Mundus apart. when 4 happens, Leona is the first one to actually talk to Nero. also haven't poked at this one much
Leona reunites with Dante after 3 but years before 4. she goes to Fortuna with Dante and she ends up discovering Vergil in Angus' labs and between his poor state and Nero's near death experience, goes ballistic and goes on the warpath
Leona doesn't reunite with Dante until nearer to 5 and doesn't meet Nero until 5. she figures out who V is and has a breakdown and just. clings. this one has the MOST angst >:]
new one i came up with today. Leona goes to Fortuna for ???? reasons and meets Nero when he's like.... pre-teen/early teens. instantly recognizes him as family and decides to move there because she doesn't want to tear him away from his home but she wants to get to know him. becomes his guardian. reunites with Dante during 4 and finds Vergil in Angus' labs during/before 4