I need someone whose imagination isn't being silly...?

I would go for medieval-type names, especially if you're using the name "Isolde". I can't think of any off the top of my head, but you could try typing medieval baby names into Google and see what happens. One thing I wouldn't do is use the name Tristan in your story. Tristan and Isolde are one of the most famous couples in history. You could always try using names from Greek Mythology, names of demi gods and stuff. Names like Demeter and Hera.