A few variants of Guinevere's name and my thoughts on them:
Velivera--sounds like you'd put it in a soap, but still mellifluous.
Jennifer--the name of half the women of my mother's generation; sounds less epic than it might once have because it's used too frequently.
Guanhamara--pretty, if a little difficult; reminds me of the character from Chronicles of the Red King.
Gwenhwyfar--the Welsh option, one of the classics.
Vanora--sounds like a pretty normal fantasy name; does not sound like Guinevere.
Ginevra--also sounds fairly normal; a little more recognizable.
Guendoloena--and her less assuming relative Gwendolen; this is also Merlin's wife's name (Geoffrey of Monmouth thought it was a good wife name, apparently).
Gaynour--I like the sound, but it would be mocked mercilessly in a modern middle school.
Guilalmier--I like it. Not as classic, maybe, but charming enough.
Wenneuereia--"Can you spell that one more time, please?" I had to check Wikipedia for the spelling of this one.
Ntzenebra--from The Old Knight, the only surviving Arthurian romance in Greek. Very cool.
G(ui/we)n(n)(i)ev(i)(e)r(e)--the closest thing we have to a standard formula of the name is this. You can add some letters or you can take away some letters, and the vibes will change, but it will still be the same in essence and pretty recognizable, unlike...
Winlogee--the coup de grâce. My feelings on this one are complicated, but I feel it can speak for itself.