They should just conclude the series with an overarching story that covers everything from Sparda's rebellion to Nero's rise and fill it to the rim with unholy levels of fan service. Full scale demonic war. Lady sacrifices herself and dies, Trish loses her demonic soul and rises to heaven. It all ends with Dante separating his soul from his body to seal away the demon world just as his father did, now with rebellion as the seal. Nero, now a seasoned hunter, takes his place and walks away into the future. (holding Kyrie's hand) ... Or not.