I’m in love with Marcelina, but at four syllables it definitely needs an (at least occasional) nickname or contraction. I’m looking for something that sounds more elegant rather than cute, though of course she’d eventually choose her own nickname when she’s able. What are your favorites?
I really love Lina, Celine, Celina, Melina, Marina, and Mara! Mari, Marcy (all spellings), and Celie seem more cute than elegant to me, and I’m not sure if Elina and Eline are meant to be pronounced like Alina and Aline, or with a more obvious “el” beginning.
Some other Marcelina nicknames i would suggest (some are a bit of a stretch) are Alina/Aline, Macy, Marie, Aria/Ari, Minnie, Ella, and Marlina.
[name_f]Mara[/name_f] and [name_f]Marcie[/name_f] are sweet, but I think [name_f]Lina[/name_f] feels most elegant.
[name_f]Mila[/name_f] and [name_u]Mina[/name_u] also came to mind
I also like [name_f]Marce[/name_f] and [name_u]Mars[/name_u] 
I like [name_f]Marcie[/name_f]!
I always thought [name_f]Minnie[/name_f] was cute for Marceline/Marcelina
my favorite is marnie 
Lina for me, and it’s definitely more elegant than Marcie etc.