My 2nd daughter is [name]Katharine[/name] and we call her by both her full name and [name]Kat[/name]. It’s really worked well and I’d like to find another girls name with a similar feel. [name]Katharine[/name] is of course classic and [name]Kat[/name] is a bit fun and unexpected. Also [name]Kat[/name] and [name]Katharine[/name] interchange really easily and I like that. (Her siblings all have 1-syllable names - in my siggy- so I’d like the next baby’s name to match [name]Katharine[/name]'s name more than the others.)
[name]Alexandra[/name] - [name]Alex[/name]
[name]Eleanor[/name] - [name]Nora[/name], [name]Ellie[/name], [name]Elle[/name], [name]Ella[/name]
[name]Bethany[/name] - [name]Beth[/name]
[name]Beverly[/name] - [name]Bev[/name]
[name]Veronica[/name] - [name]Ronnie[/name], [name]Ron[/name], [name]Vera[/name]
[name]Annabel[/name] - [name]Anna[/name], [name]Belle[/name]
[name]Juliet[/name] - [name]Julie[/name], Juls
[name]Victoria[/name] - [name]Vicky[/name], [name]Tori[/name]
I’ll add more later.
[name]Harriet[/name] - [name]Hattie[/name]
[name]Alice[/name] - Lissy
[name]Elizabeth[/name] - Lissy, [name]Elle[/name]
[name]Charlotte[/name] - [name]Lottie[/name]
[name]Scarlett[/name]- [name]Lettie[/name], [name]Carlie[/name]
[name]Adeline[/name] - [name]Ada[/name]
[name]Aurora[/name] - [name]Rory[/name]
[name]Christabel[/name] - [name]Belle[/name]
[name]Ebony[/name] - [name]Eb[/name]
[name]Emmeline[/name] - [name]Emmy[/name]
[name]Imogen[/name] - [name]Gen[/name], [name]Immy[/name],
[name]Loretta[/name] - [name]Lori[/name], [name]Retta[/name], [name]Etta[/name]
[name]Isadora[/name] - [name]Isa[/name], [name]Dori[/name], [name]Adore[/name]
[name]Estelle[/name]/[name]Estella[/name] - [name]Star[/name], [name]Stella[/name], Stellie
[name]Hope[/name] these help! I don’t know if any of them are what you are looking for!
I am a [name]Catherine[/name] (frequently called [name]Cat[/name] at work), and I have a sister named [name]Emily[/name] (frequently called [name]Em[/name] or [name]Emmy[/name]), if that helps. 
[name]Annabelle[/name]- [name]Annie[/name], [name]Belle[/name]
[name]Audrey[/name]- [name]Aud[/name]
[name]Aurora[/name]-[name]Rory[/name]
[name]Charlotte[/name]- [name]Lottie[/name], [name]Charlie[/name]
[name]Christabelle[/name]-[name]Belle[/name]
Dorothy- Dolly, Dot, Dottie
[name]Eleanora[/name]- [name]Ellie[/name], [name]Nora[/name]
[name]Elizabeth[/name]- [name]Eliza[/name], [name]Beth[/name]
[name]Hermione[/name]- Mione, [name]Mio[/name]
[name]Juliet[/name]- [name]Juli[/name]
[name]Kirsten[/name]- Kirst
Minerva- Minnie
[name]Rosemary[/name], [name]Romilly[/name]- [name]Romy[/name]
[name]Vanessa[/name]- [name]Nessie[/name]
[name]Veronica[/name]- [name]Ronnie[/name]
[name]Victoria[/name]- [name]Rory[/name]
Virginia- Ginny
[name]Violet[/name]- Vio, [name]Lettie[/name]
[name]Virginia[/name] – [name]Ginny[/name]
[name]Dorothy[/name] – [name]Dolly[/name], [name]Dot[/name], [name]Dottie[/name]
[name]Margaret[/name] - [name]Meg[/name], [name]Maggie[/name]
[name]Matilda[/name] - [name]Maude[/name]
[name]Frances[/name] - [name]Frannie[/name]
[name]Charlotte[/name], [name]Caroline[/name] - [name]Cal[/name], [name]Callie[/name]
[name]Elizabeth[/name] - [name]Bet[/name], [name]Liz[/name], [name]Bell[/name], [name]Elsie[/name]
[name]Virginia[/name] - [name]Ginny[/name]
[name]Cordelia[/name] - [name]Cora[/name]
[name]Eleanor[/name] - [name]Ellie[/name], [name]Nora[/name]
[name]Marian[/name] - [name]Mary[/name]
[name]Beatrice[/name] - [name]Bea[/name], [name]Bet[/name]
[name]Sarah[/name] - [name]Sal[/name], [name]Sally[/name]
[name]Louisa[/name] - [name]Lou[/name]
[name]Felicity[/name] - [name]Fee[/name]
[name]Josephine[/name] - [name]Josie[/name]
[name]Susannah[/name] - [name]Susie[/name]
[name]Adelaide[/name] - [name]Addie[/name], [name]Adele[/name]
[name]Miriam[/name] - [name]Mimi[/name]
[name]Lucinda[/name] - [name]Lucy[/name]
[name]Dorothy[/name] - [name]Dora[/name]
[name]Evangeline[/name] - [name]Eve[/name]
[name]Phoebe[/name]
[name]Verena[/name] - [name]Vera[/name]
[name]Jaqueline[/name] - [name]Jaq[/name]
[name]Rosemary[/name], [name]Rosalind[/name], [name]Rosamunde[/name] - [name]Rose[/name]
Good luck!
Before I even saw your sig I was thinking [name]Theresa[/name]/[name]Teresa[/name], nicknamed [name]Tess[/name]. It does work perfectly with [name]Katharine[/name]/[name]Kat[/name].
Other ideas, mostly focusing on not just nicknames but one-syllable nicks for fairly traditional/classic names:
[name]Susannah[/name] - [name]Sue[/name]/[name]Susie[/name]/[name]Suki[/name]
[name]Beatrice[/name] - [name]Bee[/name]
[name]Jessamine[/name] - [name]Jess[/name]
[name]Juliet[/name] - [name]Jet[/name]
[name]Josephine[/name] - [name]Jo[/name] (or [name]Josie[/name]/[name]Posy[/name])
[name]Elizabeth[/name] - so very many nicknames but my favourite is probably [name]Libby[/name]. [name]Kat[/name] and [name]Libby[/name] sounds so perfect to me.
[name]Rosemary[/name] - [name]Romy[/name] is my favourite, [name]Rory[/name] or [name]Rosie[/name] could work.
Honestly, [name]Theresa[/name] nn [name]Tess[/name]. lol, I would be so happy to see you use [name]Theresa[/name]! I think [name]Tess[/name] Tys0n is so adorable.
I really can’t think of any others, maybe [name]Felicity[/name] nn [name]Izzy[/name] or [name]Eleanor[/name] nn [name]Nell[/name]. Or [name]Elizabeth[/name] nn [name]Izzy[/name]/[name]Ibby[/name]/[name]Libby[/name] (love [name]Elizabeth[/name] nn [name]Libby[/name], but I’m almost afraid [name]Libby[/name]'s too sweet next to the spunky [name]Kat[/name], although it would fit in pretty well next to [name]Clare[/name]–your other daughter, right?). Or even something like [name]Susanna[/name] nn [name]Sosie[/name]. [name]Josephine[/name] nn [name]Posy[/name]?
[name]Penelope[/name] or [name]Eleanor[/name] - [name]Nell[/name]
[name]Margaret[/name] - [name]Daisy[/name]
[name]Louisa[/name]/[name]Louise[/name] - [name]Lou[/name]
[name]Beatrice[/name]/[name]Beatrix[/name] - [name]Bea[/name]
[name]Olivia[/name]/[name]Lavinia[/name] - [name]Liv[/name]
[name]Prudence[/name] - [name]Pru[/name]
[name]Dorothy[/name] - [name]Dot[/name]
Realized that this is one of the threads I’m constantly going back to and re-reading but I never reposted on it. So thanks everyone for the suggestions and if anyone has anymore I’m still interested.