Could [name_f]Millie[/name_f] be a nickname for [name_f]Marceline[/name_f]? I love [name_f]Millie[/name_f], but I feel like it isn’t substantial enough for a formal/full name, and I love [name_f]Marceline[/name_f], but don’t love [name_f]Marcy[/name_f] (I don’t hate it but just don’t love it), and my family is big on nicknames so I don’t really see us calling her by the full name all the time. I have neighbors who are calling their baby [name_m]Henry[/name_m] [name_m]Huck[/name_m], and at first it seemed a little odd, but it works? [name_m]Just[/name_m] wondering what you would think of someone doing this!
I think it’s fine. When it comes to nicknames, I feel like they really don’t have to be justified or have to make sense. [name_m]Plenty[/name_m] of names have a nonsense nickname, like [name_f]Margaret[/name_f] into [name_f]Peggy[/name_f].
I wouldn’t think twice about a [name_f]Marceline[/name_f] called [name_f]Millie[/name_f]; it absolutely works.
I think it’s fine! Another name you might consider (other than the obvious [name_f]Millicent[/name_f]; I’m assuming you’re not a fan) is [name_f]Romilly[/name_f].
I think it works as both have strong L and I sounds that connect them!
Yes, definitely. The sounds are all there, plus it fits with the sound of traditional nicknames like [name_f]Nellie[/name_f] and [name_f]Molly[/name_f] that have only a passing resemblance to their formal names.
I agree that it would work just fine 
I always think that with things like this it doesn’t really matter but once they get to school their friends will probabaly call her [name_f]Marcy[/name_f]. So then you’ll end up where you didn’t really want to be. Either name her just [name_f]Millie[/name_f] or go for a [name_f]Millie[/name_f] name. Like [name_f]Millicent[/name_f] or something similar.
I don’t think it’s too much of a stretch! Definitely not a traditional nn for it, but I can still see [name_f]Marceline[/name_f] “[name_f]Millie[/name_f]” and think that it makes sense. 
I think it works! Honestly I like [name_f]Millie[/name_f] better as a nickname then [name_f]Marcy[/name_f], and to me [name_f]Millie[/name_f] totally fits.
Thanks everyone! I feel much better about this now!