If you like Guinevere (I love it!), why not nickname Winnie? Is that too sweet? I think it's amazing.
My favorite pairs are Guinevere and Lorelei, Guinevere and Rosalind, and Sylvia and Eleanor.
Thank you so much for all the wonderful suggestions! I'm not sure why I didn't think of more Guinevere nicknames - I'll blame it on pregnancy brain, eh? I absolutely ADORE Ginny, especially, though they are all nice.
I like that Lucy was suggested - my husband's been pushing hard for that one. Do you think it stands alone, or does it need a more formal name behind it?
Caoimhe is one that I'm absolutely smitten with, but my mister has turned it down more times than I can count! Bah!
Some that I loved (that surprised me!):
-Vivian (what is it that's so elegant about Vs?)
-Melisande (absolutely dreamy - my husband is pretty nuts about this one already, so it may go on the list. The possibly Ana nickname doesn't hurt, either.)
-Rosalind (this one just brings to mind a beautiful rosy-cheeked, wholesome-looking girl for me)
I love Fiona, too, but alas one of my husband's coworkers just named their baby that...it would look a bit like name poaching. :)
Thanks all for the lovely suggestions.
One last thought - would it be strange to have two children with an "M" name and one without? Do you think the other child would feel weird or left out because of that? I'm not crazy about all names of one letter in a sibling set, but there are a few M names in here that I really like!
Lucy can definitely stand on its own.
As for having multiple children with the same initial and others without, I just wanted to chime in from personal experience that it doesn't really matter. In my family of 3 girls, we are Lisa Eileen, Laura Colleen, and Alice Suzanne. No one ever made a big deal out of the fact that my middle sister and I have similar names and the youngest didn't so I wouldn't worry too much. :)