I know a little girl named
Thea. To me the name is subtantial enough to stand on its own. But there certainly are other gorgeous names that it could be a nn for:
TheodoraTheodosiaTheophila ("thee-AH-fil-a")
Theophania Dorothea /
DorothyCynthiaAltheaAthenaXantheCalantheAmaranthe
IantheIolantheTheresa
Some of these are a bit of a stretch. But nns aren't scientific to me. You can get as creative as you like with a nn and it's a plus in my book.