I’m also interested to see what they choose! From what I remember, there was talk when [name_m]Spurgeon[/name_m] was born that they were going to call him [name_m]Elliot[/name_m], but it appears that never did happen. I also vaguely remember some discussion about how [name_f]Jessa[/name_f] chooses the first names and [name_m]Ben[/name_m] chooses the middle names (although they flipped it for Spurgeon). [name_m]Ben[/name_m] seems to go for theologian (specifically Calvinist) names as boy middle names, and I think [name_f]Jane[/name_f] is after [name_f]Lady[/name_f] [name_f]Jane[/name_f] [name_m]Grey[/name_m]? [name_f]Elliana[/name_f] is a bit of a wildcard, but I assume they chose it for the meaning (though the double ‘l’ has always puzzled me a bit).
[name_f][/name_f]
I can see [name_f]Pearl[/name_f] for a girl, especially in reference to “pearl of great price”. It does feel slighly close to [name_m]Spurgeon[/name_m] and [name_f]Fern[/name_f] to me, but this is a family centered around 19 kids starting with J so… 
[name_f][/name_f]
On to my suggestions –
[name_f][/name_f]
For girls, there’s definitely an established(?) botanical theme, but I could see them branching out into nature-adjacent choices. Along the same lines as [name_f]Pearl[/name_f], something like [name_f]Ruby[/name_f][name_f][/name_f] or [name_f]Opal[/name_f][name_f][/name_f]. I would also love to see them with an [name_f]Olive[/name_f][name_f][/name_f] (biblical imagery!). A few others for first names: [name_f]June[/name_f][name_f][/name_f] / [name_f]Junia[/name_f][name_f][/name_f], [name_f]Dove[/name_f][name_f][/name_f], [name_f]Stella[/name_f][name_f][/name_f], [name_f]Eden[/name_f][name_f][/name_f], [name_f]Calla[/name_f][name_f][/name_f], [name_f]Phoebe[/name_f][name_f][/name_f]. Wildcard pick is [name_f]Piper[/name_f][name_f][/name_f] after [name_m]John[/name_m] [name_f]Piper[/name_f]. I also think they might use this as a middle for boy or girl.
[name_f][/name_f]
For boys, I am shocked[name_f][/name_f] there is not already a [name_m]Theodore[/name_m][name_f][/name_f] in the family. I especially like it for the Seewalds as [name_m]Theodore[/name_m] Beza was a successor to [name_m]John[/name_m] [name_m]Calvin[/name_m]. Speaking of which, [name_m]Calvin[/name_m][name_f][/name_f]? I assume they’ve probably already considered it and it’s too “on the nose”. Maybe as a middle? Other first names: [name_m]Arthur[/name_m][name_f][/name_f], [name_m]Francis[/name_m][name_f][/name_f] (maybe too Catholic, but [name_m]Francis[/name_m] Schaeffer was a Calvinist apologist. How cute would [name_m]Frankie[/name_m] be?), [name_m]Edward[/name_m][name_f][/name_f], [name_m]Simon[/name_m][name_f][/name_f], [name_m]Owen[/name_m][name_f][/name_f] (after [name_m]John[/name_m] Owen). Wildcard is [name_m]Abraham[/name_m][name_f][/name_f] or [name_m]Bram[/name_m][name_f][/name_f] after [name_m]Abraham[/name_m] Kuyper.
[name_f][/name_f]
Middles are fun, because I feel like I definitely have an idea of where they’re choosing from (excepting Elliana).
[name_f][/name_f]
Middle name picks:
[name_f][/name_f]
[name_m]Anselm[/name_m]
[name_f][/name_f][name_m]Luther[/name_m]
[name_f][/name_f][name_m]Knox[/name_m]
[name_f][/name_f][name_m]Hodge[/name_m]
[name_f][/name_f][name_m]Cornelius[/name_m]
[name_f][/name_f]Kuyper
[name_f][/name_f][name_m]Dietrich[/name_m]
[name_f][/name_f][name_m]Caspar[/name_m]
[name_f][/name_f][name_m]Gresham[/name_m]
[name_f][/name_f]
[name_f]Elizabeth[/name_f] / [name_f]Elisabeth[/name_f] (probably don’t need another [name_f]El[/name_f]- middle name, though)
[name_f][/name_f][name_f]Katharina[/name_f]
[name_f][/name_f][name_f]Pepper[/name_f]
[name_f][/name_f][name_f]Alice[/name_f]
[name_f][/name_f][name_f]Moon[/name_f]
[name_f][/name_f][name_f]Dorothy[/name_f] / [name_f]Dorothea[/name_f]
[name_f][/name_f][name_f]Verity[/name_f]
[name_f][/name_f][name_f]Aletheia[/name_f]