@Jobslayer
I don't think there will be some big arc for Itsuki.
For Itsuki, I think her role is getting close to set. I think she has an unresolved personal issue with her father or in trusting men due to her father... Or something like that. (I just remember there was a time Itsuki said something about trusting boys and Ichika said she was still thinking that way probably because of their dad). But I don't think it will take a whole arc.
She reminded me in the water park chapter that she does have a special relationship with Fuu and they are able to talk and thought maybe she could be the girl at the bell at the end if Negi wants to... But maybe it's just best two people have that close relationship and not have to become more than that.
Ichika may have taken a sort of big sis after their mom died but Itsuki was the one that said she would be their mom and I feel she does act that way to her sisters in the support she gives each. Maybe she has at this point become kinda is like that to Fuu too in their special relationship since he doesnt have a mom either.