Can I just add that I know sisters Natalie and Abigail and I think they combine really well! Tallie is my favorite nn for Natalie.
Millicent (Millie) - is heavily old lady to me, to the extent that it’s back in, it’s in an old-lady-chick way, much more so than Abigail.
Eleanor (Ellie) - I like Eleanor, and it does meet what you’re looking for. Nell/Nellie and Nora are two more nn options.
Charlotte (Lottie) - I love Charlotte and Lottie! Charlotte is a forever wearable name that is “in” right now, it is a little trendy in that respect, much like Abigail. FYI, I’m saying that as praise overall, not as a drawback : D.
Sarah - I agree with a pp that it had a major moment of glory in the 1980s, but it’s not like it’s going out of style horribly. I still like it and think it together with Abigail is nice for being a biblical set. The nn’s Sadie and Sally are great with Abigail (Abby?)
Alice - Alice is a great name that to me is still a little more in the vintage-chic than timeless, though it’s close. I think though you may want to avoid another A name if you’re having more kids? Just a thought.
Camilla - Pretty, but, to me, not exactly timeless. To me, doesn’t pair as well with Abigail as some others.
Katherine - great, definitely classic name that I like with Abigail. Kate, Katie, Kit, Kitty are all cute nn’s.
Sophia (just added) - pretty and timeless in its way, though very in right now. I know this is weirds for name, but, for me, feels much more exotic and sensual than Abigail, which to me is a much more squeaky-clean, All-American name.
Some more suggestions I would love to see with Abigail that I think are in your ball park:
[b]Julia /b
Miriam (Mimi, Mira, Miri - another Biblical name)
Mary (Molly, Mamie, Mae - I think Mae as a middle name would be odd here, but good as a nn for Mary)
Margaret (Maisie, Maggie, Margot, Mae, Mae would work as a middle or a nn - Maggie Mae adorable if you don’t mind the song!)
Violet (Vi, Lettie) - just love this with Abigail! It is a bit “trendier” than the names above, but, is a name with great history so while it’s “in” now it’s not exactly tied to this period in time).
Josephine (Josie)
Lily - quite, quite popular right now, but, so is Abigail! And like Abigail, a solid history of use. I could see nn Millie btw by rhyming if that’s something you really want. Lily Mae = Millie Mae
Madeline - similar feedback to Lily and Violet, Maddie is quite the rage but cute and wonderful I think for Maddie Mae.
Harriet (Hallie, Etta, Ettie) - Harriet is much clunkier and vintage chic than most of these names, more like Alice in that regard, I don’t actually love the name, but if you do - I love several nn’s for it and think it pairs wonderfully with Abigail and Mae.
Phoebe - so cute with Mae, Biblical too.
Olivia - like Sophia, an old, historied name that is particularly popular right now (like Abigail). As with all the names above, I really love two vowel names together. Nns Liv and Livvy are so great - Livvy Mae is great!
Lucy - Lulu or Lula - so cute, sweet, and fresh, sounds natural with Abigail and with Mae to me.
Eve - Biblical and classic, Evie Mae is so sweet. Much simpler than Abigail but I think it works.
Iris - more on the vintage chic side of things, but I like it with Abigail. Similar feel to Alice to me, but you get separate initials.
I’m sure there are more! Which way are you leaning? All best : D!