[name_f]Cici[/name_f] / [name_f]Cece[/name_f] / CC is one that hasn’t been mentioned yet!
I’ve also heard [name_f]Stina[/name_f] for [name_f]Christina[/name_f], but I feel like it could work for [name_f]Christine[/name_f] too.
[name_f]Issy[/name_f] and [name_f]Sissy[/name_f] could also be options (though [name_f]Sissy[/name_f] might be better as an at-home nickname).