Ramona is my all-time favorite girl name! I just have to get my husband to love it!
Similar to June, what about Junia or Juniper (love it and nn Juni)
Sutton is a cool middle name!
Some other ideas that I think go with your other awesomely named kiddos:
Eloise, Beatrix, and Foster is a gorgeous sibset! I love Harriet, Ramona, and Maxine--all beautiful, classic choices that nicely complement your other children's names. Rosamund would also work well.
As far as other first names are concerned, I would suggest:
Alice, Audrey, Camilla, Cecily, Charlotte, Clementine, Genevieve, Georgiana, Hazel, Isadora, Josephine, Juliet, Louisa, Lucille, Matilda, Penelope, Scarlett, Sylvie, Violet
Ottilie Maxine - I'm not keen on Ottilie I like Ottoline but I find this name slightly off peak. However the name does work well with Maxine.
Ottoline June - I love this choice, I like Ottoline is very spunky and the ending just works. June works beautifully with Ottoline.
Harriet Maxine - Harriet is such a classy choice, she's grown on me so much. Yet I dislike with Maxine the two names have a strange flow.
Harriet Sutton - I think Harriet is great obviously and I love Harriet with the boyish trendy Sutton. Great combination!
Guinevere Maxine - Lovely combination Guinevere is very majestic and pretty. I also think Maxine works nicely with Guinevere! Good choice.
Yet I wonder whether Guinevere is too long and looks out of place with Eloise, Beatrix and Foster.
Theodora June - I adore Theodora she has this rural folk sounding charm. Theodora looks elegant and works lovingly with June. Great combination yet I do feel that Theodora is far too long for Eloise, Beatrix and Foster.
Ramona - I'm unsure on Ramona sometimes I adore this choice I love her cheerfulness. On the other hand I dislike the 'Mona' part of the name as it reminds me of the unfavourable trait of moaning. I must prefer Romina. Which I think works with Eloise, Beatrix & Foster.
Maxine - Isn't a favourite of mine. I think Maxine sounds slightly 90s. Not my thing.
Agatha - She has a very vintage feel. I like the nickname Aggie as well, I also think Agatha works well with Eloise, Beatrix & Foster.
Araminta - I love this choice, she feels very fresh and minty. Araminta is really sweet and spunky. Yet I don't think she works with Eloise, Beatrix & Foster the name just doesn't have the same feel. Their too different in style.
Rosamund - I love this choice, she's so fresh and elegant. Very humble and sounds perfect with Eloise, Beatrix & Foster.
Romilly - I dislike this choice, too me Romilly feels strange.