I like many of your options. I ended up voting for [name_u]Ira[/name_u] with [name_f]Willow[/name_f]. I think they’re a good, slightly offbeat pairing.
[name_f]Willow[/name_f] and [name_u]River[/name_u] seem like an obvious sibset and it definitely calls to me, but I also love [name_f]Willow[/name_f] and [name_m]Atlas[/name_m], [name_f]Willow[/name_f] and [name_m]Felix[/name_m], and [name_f]Willow[/name_f] and [name_m]Julian[/name_m].
I find [name_f]Willow[/name_f] & [name_u]River[/name_u] almost a little too obvious in a matchy nature theme kind of way, but if you love it then that could be cute.
My faves would be [name_f]Willow[/name_f] & [name_m]Luke[/name_m], [name_m]Julian[/name_m] and [name_m]Felix[/name_m].
I went for [name_f]Willow[/name_f] and [name_m]Felix[/name_m]. In my opinion they seem to match together better. I also like [name_m]Otto[/name_m].
Other names that may work for you would be:
[name_m]Oscar[/name_m]
[name_m]Zachary[/name_m]
[name_m]Asher[/name_m]
[name_u]Noah[/name_u]
[name_u]Aspen[/name_u]
[name_m]Casper[/name_m]
[name_m]George[/name_m]
[name_m]Milo[/name_m]
[name_u]Rowan[/name_u]
[name_u]Evan[/name_u]
[name_u]Kieron[/name_u]
[name_m]Lucas[/name_m]
All are such good choices! My top favorites would probably be [name_m]Felix[/name_m] (which I voted for), [name_m]Atlas[/name_m], [name_u]River[/name_u], [name_m]Otto[/name_m], and [name_m]Julian[/name_m].
I agree [name_f]Willow[/name_f] & [name_u]River[/name_u] sound nature-y but I think that could be very sweet!
[name_f]Willow[/name_f] and [name_u]River[/name_u] I find a bit too matchy and really locks you in to a theme for future siblings (if they are in your plans!)
I think [name_f]Willow[/name_f] and [name_u]Ira[/name_u] is absolutely gorgeous - both are distinctive, calm, offbeat but elegant names
[name_u]Ira[/name_u] deserves so much more attention!