In the book Beyond Jennifer and Jason, Madison and Montana, there are a bunch of lists in which names are grouped together according to some common trait (like biblical names, or Western cowboy names, or names from Disney movies). I couldn't find any lists that had both Isabel/Isabelle and Caleb in them, but here are some of the names each one was grouped with:
Caleb-- New England names: Isaiah, Noah, Tobias, Jeremiah, Everett Biblical: Aaron, Isaac, Jared, Elijah, Jonah, Samson (plus most of the names above) Fitting In/Standing Out(names that are a little less popular but not too off the wall): Adrian, Carson, Duncan, Holden, Julian, Leo, Lucas, Owen, Seth, Trevor
Whew! I hope this helps a little. My husband and I were just talking about the same thing a few days ago--our nine-month old daughter is named Clio, and when the next one comes along I haven't a clue what he or she will be named--nothing seems to sound right with it. Well, good luck(and congrats:))!