I’m in love with [name_f]Marcella[/name_f] as a whole name, but I feel like it’s the sort of name that would immediately get shortened by everyone who met one. I’m also not a fan of [name_f]Marcy[/name_f] or [name_f]Ella[/name_f], which seem like the intuitive nns. Would [name_f]Maisie[/name_f] work?
Thanks 
[name_f]Mara[/name_f]
[name_f]Mari[/name_f]
[name_f]Marly[/name_f]
[name_f]Marla[/name_f]
[name_f]Cella[/name_f]
[name_f]Celie[/name_f]
[name_u]Mars[/name_u]
[name_f]Marnie[/name_f]
[name_f]Marlo[/name_f]
[name_m]Maro[/name_m]
[name_f]May[/name_f]
[name_f]Mamie[/name_f]
[name_u]Mel[/name_u]
[name_f]Mellie[/name_f]
[name_f]Mala[/name_f]
[name_f]Mallie[/name_f]
And yeah, [name_f]Maisie[/name_f] could work!
THought of more:
[name_f]Elsie[/name_f]
[name_f]Elle[/name_f]
Cel
Maz
[name_f]Masha[/name_f]
[name_f]Marcie[/name_f], [name_f]Cella[/name_f]/[name_f]Cilla[/name_f], and [name_f]Ella[/name_f] were the first to come to mind to me. [name_f]Maisie[/name_f] is a bit of a stretch, but I think you could pull it off.
It’s not too big of a stretch but I really think you should just communicate that she will use her full name. I hate it when people run with nicknames that haven’t been signed off by the parents or the child. This coming from a [name_f]Christina[/name_f] that shudders when anyone takes the liberty to call me [name_f]Chrissy[/name_f].
I think you could do [name_f]Marcella[/name_f] in full if you wanted. I’m seeing a lot more kids/young adults going by their full names than in older generations. I know at least one [name_m]Thomas[/name_m], [name_m]Patrick[/name_m], [name_f]Caroline[/name_f], [name_m]Christopher[/name_m], [name_m]Harrison[/name_m], and others that go by their full names. [name_f]Marcella[/name_f] is such a beautiful name.
[name_m]Just[/name_m] introduce her as [name_f]Marcella[/name_f] with no nickname.
If you don’t mind it shortened, I think Cellie or [name_f]Cella[/name_f] are adorable nicknames.
[name_f]Maisie[/name_f] is definitely doable - I’ve always liked [name_f]Mercy[/name_f] for [name_f]Marcella[/name_f], or [name_f]Mary[/name_f].
I had a character named [name_f]Marcella[/name_f] and her bestie called her all sorts of nns: [name_f]Marcie[/name_f], [name_f]Marcie[/name_f] [name_f]Mae[/name_f] (her middle was [name_f]Mae[/name_f]), Mazzy, [name_f]Cella[/name_f], Cellie, Markella, Kella, [name_m]Marc[/name_m]. To name a few. My favorite was Mazzy. I think [name_f]Maisie[/name_f] works, especially if you use a name that ends in the -ie sound for the middle. For example: [name_f]Marcella[/name_f] [name_f]Daisy[/name_f], [name_f]Marcella[/name_f] [name_f]Chloe[/name_f], [name_f]Marcella[/name_f] [name_u]Paisley[/name_u], etc.
I [name_u]LOVE[/name_u] [name_f]Marcella[/name_f] and [name_f]Marceline[/name_f]! I think you could definitely pull or [name_f]Maisie[/name_f].
I would never butcher it
[name_f]Marly[/name_f] was the first nn I thought of for [name_f]Marcella[/name_f].
I knew a guy whose name was [name_m]Marcel[/name_m] and he went by [name_u]Mars[/name_u], which is a kind of out there but also cute and definitely unique nickname. 