I like a lot of these with [name]Boston[/name] and [name]Piper[/name]!
[name]Belle[/name] - I think this works really well with [name]Belle[/name], although quite a bit more feminine. I noticed [name]Bellamy[/name] further down on your list, which is technically a bit more unisex, like [name]Piper[/name], even though they both have very girly feels to them–maybe [name]Bellamy[/name], nn [name]Belle[/name]?
[name]Arden[/name] - I like this with [name]Boston[/name] and [name]Piper[/name], too, but not as much as [name]Belle[/name]/[name]Bellamy[/name]. Still, it’s nice.
[name]Primrose[/name] - I was just thinking about [name]Primrose[/name], nn [name]Posey[/name], today! I prefer it spelled [name]Posey[/name], but I still love it. I think [name]Posey[/name] is adorable, and lovely next to [name]Piper[/name].
[name]Sosie[/name] - very cute, and adorable next to [name]Piper[/name] and [name]Boston[/name], but it seems kind of nicknamey. Have you thought about using a fuller name, with the nn [name]Sosie[/name]? My first idea was [name]Sophia[/name], nn [name]Sosie[/name], but I think something like [name]Shoshana[/name] could work, too…
[name]Meadow[/name] - cute, and definitely fits in. I definitely love [name]Meadow[/name], but I’ve always found it too GP-ish to use in real life.
[name]Remy[/name] - I actually considered this to honor my grandmother, too! That kind of amuses me. It’s a bit too masculine for me, though–I had considered [name]Remy[/name] for a boy.
[name]Bellamy[/name] - I think this is actually really perfect with [name]Boston[/name] and [name]Piper[/name]. It’s a lovely full name, and the nns [name]Belle[/name], [name]Bella[/name], [name]Amy[/name], [name]Amie[/name] (ah-MEE), and [name]Emmy[/name] could all be adorable choices, and all work well with [name]Boston[/name] and [name]Piper[/name]…
[name]Zanna[/name] - lovely… you might also consider [name]Sanne[/name]? [name]Said[/name] like sah-neh, I believe? Other “Z” names, like [name]Zara[/name] and [name]Zinnia[/name], seem like they could work with your sibset, too.
[name]Zadie[/name] - cute. I think both this and the more mainstream [name]Sadie[/name] could work with [name]Boston[/name] and [name]Piper[/name].
I also think an [name]Addy[/name] name might also work nicely with your children–maybe [name]Adelia[/name], [name]Adela[/name], or Adalet? [name]Ada[/name] and [name]Adelaide[/name] don’t feel modern enough, and I think [name]Addison[/name] is too unisex…
Good luck!