I've been really into
Greta lately, I think it is fab. Spunky, unexpected, approachable, intuitive to the full name, and it will age well as it sounds sweet on a small child but not immature on an adult.
I think
Madge and
Midge are terribly unattractive.
Mae is cute but I see it as a name in its own right rather than a nickname. Ditto
Margo.
I'm not
Meg's biggest fan but I know a couple IRL and it works well IMO. It ages well and is very friendly. I know a 20 year old Meggie and I think it sounds ridiculous. Possibly could be confusing at family gatherings having a
Peggy and Meggie around?
Maisie is adorable. If it isn't going to be an all the time nickname, I think it is a lovely option.
I like
Mamie prn mam-EE but may-mee reminds me of 'maim'
Greta or
Maisie would be my picks.