My husband is really liking the name [name_f]Maisie[/name_f]. I’m not in love with it though. Are there any more formal names that [name_f]Maisie[/name_f] could be a nickname for?
Usually: [name_f]Margaret[/name_f], [name_f]Margo[/name_f]
Other options I like (favorites in bold):
[name_f]Maeve[/name_f]
[name_f]Marissa[/name_f]
[name_f]Marine[/name_f]/[name_f]Marina[/name_f]
[name_f]Mabel[/name_f]
[name_f]Marigold[/name_f]
[name_f]Matilda[/name_f]
[name_f]Martha[/name_f]
[name_f]Marcella[/name_f]
[name_f]May[/name_f]/[name_f]Mae[/name_f]
[name_f]Maribel[/name_f]
[name_f]Magnolia[/name_f]
[name_f]Marjorie[/name_f]
[name_f]Malia[/name_f]
[name_f]Mariela[/name_f]
[name_f]Marietta[/name_f]
[name_f]Marita[/name_f] (pronounced Mah-ree-tah not ma-rita)
[name_u]Meredith[/name_u]
I think it can work as a full name and [name_f]Margaret[/name_f] works really well too.
[name_f]Marisol[/name_f]
[name_f]Mariposa[/name_f]
[name_u]Marion[/name_u]
[name_f]Marta[/name_f]
[name_f]Michaela[/name_f]
[name_f]May[/name_f]
[name_f]Miriam[/name_f]
I love [name_f]Maisie[/name_f]!
[name_f]Amaryllis[/name_f]
[name_f]Marianne[/name_f]
[name_f]Mariana[/name_f]
[name_f]May[/name_f]
[name_u]Damaris[/name_u]
[name_f]Mary[/name_f]
[name_f]Rosemary[/name_f]
[name_f]Maelys[/name_f]
Madalena
[name_f]Maia[/name_f]/[name_f]Maya[/name_f]
[name_f]Mara[/name_f]
Messalina
[name_f]Euphemia[/name_f]
Carisimma
[name_f]Esme[/name_f]
[name_f]Maryam[/name_f]
I think [name_f]Maisie[/name_f] can stand on its own rather than as a nickname if you did want to consider that. Another name is [name_f]Mavis[/name_f], nn [name_f]Maisie[/name_f].
My first thought was [name_f]Margaret[/name_f]. After all, ‘[name_f]Daisy[/name_f]’ is considered a diminutive of [name_f]Margaret[/name_f], and swapping out that ‘D’ for an ‘M’ makes a lot more sense! 
There are so many great suggestions above, so, to avoid repeating them all, let me suggest something a little different. Have you considered combining her first and middle name to get [name_f]Maisie[/name_f] by the sounds? For example, [name_f]Mary[/name_f] [name_f]Hazel[/name_f] or [name_f]Matilda[/name_f] [name_f]Zoe[/name_f].
[name_f]Marguerite[/name_f] nickname [name_f]Maisie[/name_f] has always been the favourite for me.
I love the suggestion of [name_f]Marguerite[/name_f]!