In 1-2 years time, Ikaruga / Katsuragi will graduate, Yagyuu being the leader, Hibari quit ninja academy* or be Yagyuu's wife / leech, Asuka the new Daidouji.
Replace Daidouji? Daidouji will never graduate!
I'd think Asuka would take up the leader position considering her grandfather was a legendary ninja, she'll want to live up to her descendant's fame. I don't think Yagyuu will do anything other than perv on Hibari, even if she did quit which I don't think is likely.
I'm really surprised to see fanart of that armour costume. It's kinda cool but I would've thought she'd wear maybe a blue colour.