Clara - quite like it. Perhaps a bit plain in comparison to Arabella and Leonora
Francesca - love it. And goes perfectly with your existing children.
Victoria - really like it but not its common shortenings (Vicky and Tori. Quite like Toria)
Rosalia - this almost crosses a line into 'too fancy' for me. Rosalie, Rosaline, Rosalind, Rosanna, Rosemary seem much more useable to me. Not sure why.
Tristan - very 'posh' but quite like it.
Caspian - I have a little relative called Caspian so I love it. Surprised so many people agree with you that it's close to Jasper. I don't see the link between the two names??
Felix - love it.
Louis - not keen. Some people say it Loo-ee and some say Lew-is. I like Lewis but would spell it Lewis and think that doesn't go with your children's names. Loo-ee does go but I don't really like it.
I would choose Francesca and either Caspian or Felix. All three are perfect with Arabella, Jasper and Leonora in my opinion.
Other ideas:
Adeline, Aurelia, Anastasia, Beatrice, Charlotte, Cassandra, Clarissa, Delilah, Daphne, Eleanor, Evangeline, Emilia, Felicity, Florence, Frances, Georgina, Gabriella, Genevieve, Harriet, Henrietta, Hermione, Isadora, Imogen, Isolde, Juliette, Jacinta, Jemima, Lydia, Lorelai, Larissa, Miranda, Madeleine, Matilda, Natalie, Nadia, Natasha, Ottilie, Ophelia, Octavia, Philippa, Penelope, Phoebe, Romilly, Ramona, Rowena, Saskia, Susannah, Stella, Tallulah, Tabitha, Tamara, Vanessa, Verity, Violet, Xanthe, Zara
Arthur, Albert, Benjamin, Benedict, Charles, Christian, Dominic, David, Edward, Elliott, Frederick, Francis, George, Gregory, Hugo, Hugh, Jeremy, Jude, Kit, Lysander, Montgomery, Marcus, Nicholas, Nathaniel, Owen, Oscar, Patrick, Quentin, Rupert, Rafferty, Solomon, Sidney, Thomas, Timothy, Vincent, Victor, William, Wildred, Xavier, Zachariah