My Hubby and I are due any day with our second child, a girl and have agreed on middle name [name_f]Joy[/name_f], as it bears significance for us. Our two year old son’s name is [name_m]Beckett[/name_m] so would love a name that compliments his name - strong, somewhat unique and grows well as they become adults. My number one choice is [name_f]Willa[/name_f] [name_f]Joy[/name_f], while my hubby’s is Mavie [name_f]Joy[/name_f] or [name_f]Hallie[/name_f] [name_f]Joy[/name_f]. Would love your input 
I think [name_f]Willa[/name_f] fits your criteria the best- Mavie and [name_f]Hallie[/name_f] don’t fel like they would age as well in my opinion. Plus, it goes great with [name_m]Beckett[/name_m]!
Oh, I love [name_f]Willa[/name_f] [name_f]Joy[/name_f], and I think it pairs the best with [name_m]Beckett[/name_m] of the three names you’ve listed. Have you considered [name_f]Mavis[/name_f]? It feels stronger and like it would age better than Mavie, but is very similar.
I think [name_f]Willa[/name_f] and [name_f]Hallie[/name_f] both go beautifully with [name_m]Beckett[/name_m] and also with the middle name [name_f]Joy[/name_f] so you can’t go wrong. Mavie is too nicknamey for me sorry.
[name_f]Willa[/name_f] [name_f]Joy[/name_f] is beautiful! It goes very nicely with [name_m]Beckett[/name_m]!
I like [name_f]Willa[/name_f]!
I love [name_f]Willa[/name_f] [name_f]Joy[/name_f]! Mavie seems incomplete. Maybe [name_f]Maven[/name_f] nicknames Mavie?
I like [name_f]Hallie[/name_f] [name_f]Joy[/name_f]! [name_f]Hallie[/name_f] and [name_m]Beckett[/name_m] make great sibling names.
Another vote for [name_f]Willa[/name_f]! [name_f]Hallie[/name_f] is also a nice pick with [name_m]Beckett[/name_m] and would be my second choice (though I do like the sound and nickname-y concept of Mavie).
Alright, going out on a limb here, what about [name_f]Millie[/name_f] [name_f]Joy[/name_f]? I feel like, in addition to fitting well with [name_m]Beckett[/name_m], it kind of combines sounds and qualities of the three names in question. It’s a compromise, basically.
Mavie + Willa + Halie = [name_f]Millie[/name_f]
[name_f]Willa[/name_f] is my favorite of the three but I love the suggestion of [name_f]Mavis[/name_f]!
[name_m]Beckett[/name_m] and [name_f]Hailey[/name_f] [name_f]Joy[/name_f].
I prefer the [name_f]Hailey[/name_f] (or [name_u]Haley[/name_u]) spelling, pronounced HAIL ee, pronounced with a long A sound. [name_f]Hallie[/name_f] I think is pronounced more like [name_f]Sally[/name_f] with a short A sound.
I like [name_f]Hallie[/name_f] by a mile with [name_m]Beckett[/name_m]. [name_f]Willa[/name_f] seems insubstantial too me, particularly with sweet but short [name_f]Joy[/name_f]. [name_f]Willa[/name_f] [name_f]Joy[/name_f] seems better to me as a double-barreled first. To me, [name_f]Willa[/name_f] is kind of southern and country sounding, whereas [name_f]Hallie[/name_f] is more evocative and sophisticated;I prefer that with [name_m]Beckett[/name_m]. I also like the suggestion of [name_f]Millie[/name_f].
Good luck!
I adore [name_f]Hallie[/name_f]! [name_m]Beckett[/name_m] and [name_f]Hallie[/name_f] make a dream sibset!