[name_f]My[/name_f] favorite of the suggestions is [name_f]Eleanor[/name_f] with the nn [name_f]Nell[/name_f], but I also think [name_f]Adeline[/name_f] is such a good match with [name_u]Benjamin[/name_u] and [name_m]William[/name_m] (also less popular, if that matters), and both [name_f]Elle[/name_f] and Delle/Della are super cute! Also have to plug [name_f]Helen[/name_f], which is a personal favorite of mine.
I think [name_u]Abby[/name_u] already covered the best classic names that could get you to Elle/Nell (only others I could think of were maybe [name_f]Annabel[/name_f] and Anneliese), so here are some ideas of other classics with sweet but spunky nicknames (which is the vibe I get from Elle/Nell):
[name_f]Margaret[/name_f] - [name_f]Maisy[/name_f], [name_f]Daisy[/name_f], [name_f]Pearl[/name_f], [name_f]Midge[/name_f], [name_f]Margot[/name_f] (or the more common [name_f]Maggie[/name_f], Meggie)
[name_f]Catherine[/name_f] - [name_u]Kit[/name_u], Wren/Rin
[name_f]Amelia[/name_f] - [name_f]Millie[/name_f]
[name_f]Josephine[/name_f] - [name_f]Posy[/name_f], [name_u]Josie[/name_u], [name_f]Sosie[/name_f]
[name_f]Therese[/name_f] (some might argue “classicness” of this one I suppose) - [name_f]Essie[/name_f], [name_f]Tess[/name_f], [name_f]Rosy[/name_f], [name_u]Reese[/name_u]
[name_f]Genevieve[/name_f] - [name_f]Vivi[/name_f], [name_f]Evie[/name_f], [name_f]Neve[/name_f]
[name_f]Caroline[/name_f] - [name_f]Callie[/name_f], [name_f]Caddie[/name_f]