We want to go with
I love [name_f]Margot[/name_f] [name_f]Isabelle[/name_f]! [name_f]Margot[/name_f] [name_f]Catherine[/name_f] is nice and perfectly usable, but to me [name_f]Margot[/name_f] is a heavier name and [name_f]Isabelle[/name_f] is lighter and I think they complement each other really well.
I think many of the -lia names would sound beautiful with [name_f]Margot[/name_f]. My favourite would be [name_f]Margot[/name_f] [name_f]Cecilia[/name_f].
[name_f]Margot[/name_f] [name_f]Juliet[/name_f]
[name_f]Margot[/name_f] [name_f]Georgiana[/name_f]
[name_f]Margot[/name_f] [name_f]Rosalind[/name_f]/-mund/-lie
I’m totally blanking on names right now, I’ll come back later with more ideas.
Thank you! [name_f]Do[/name_f] you think [name_f]Margot[/name_f] [name_f]Isabelle[/name_f] is a little too [name_m]French[/name_m] though? Neither of us have any [name_m]French[/name_m] roots.
[name_u]Love[/name_u] [name_f]Margot[/name_f] [name_f]Cecilia[/name_f]! Not sure I could get my husband on board with it but the combo is really pretty thank you!
[name_f]Margot[/name_f] [name_f]Caroline[/name_f]
[name_f]Margot[/name_f] [name_f]Jocelyn[/name_f]
[name_f]Margot[/name_f] [name_f]Roselyn[/name_f]
[name_f]Margot[/name_f] [name_f]Sophia[/name_f]
I agree that [name_f]Margot[/name_f] needs a “lighter” middle name.
[name_f]Margot[/name_f] [name_f]Celia[/name_f]
[name_f]Margot[/name_f] [name_f]Emmeline[/name_f]
[name_f]Margot[/name_f] [name_u]Evelyn[/name_u]
[name_f]Margot[/name_f] [name_f]Vivienne[/name_f]
[name_f]Margot[/name_f] [name_f]Adeline[/name_f]
[name_f]Margot[/name_f] [name_f]Lucia[/name_f]
[name_f]Margot[/name_f] [name_f]Ophelia[/name_f]
[name_f]Margot[/name_f] [name_f]Amelia[/name_f]
[name_f]Margot[/name_f] [name_f]Fiona[/name_f]
I don’t think [name_f]Margot[/name_f] [name_f]Isabelle[/name_f] is too [name_m]French[/name_m] at all. I think it is quite lovely. I also like c@29’s suggestion of [name_f]Margot[/name_f] [name_f]Emmeline[/name_f], [name_f]Margot[/name_f] [name_f]Vivienne[/name_f], [name_f]Margot[/name_f] [name_f]Adeline[/name_f], and [name_f]Margot[/name_f] [name_f]Lucia[/name_f].
Oh I love [name_f]Margot[/name_f] [name_f]Lucia[/name_f]! So pretty. I’ll have to ask my husband about that one tonight 
I love a lot of the others but a lot of them start with either A or E which I want to try and stay away from.
[name_f]Margot[/name_f] [name_f]Ophelia[/name_f] reminded me that I had [name_f]Margot[/name_f] [name_f]Olivia[/name_f] on my list at one point and completely forgot about it! What do you guys think of it? Too many O’s?
Another vote for [name_f]Margot[/name_f] [name_f]Lucia[/name_f]!
[name_f]Margot[/name_f] [name_f]Isabelle[/name_f] is perfection!
Maybe:
[name_f]Margot[/name_f] [name_f]Christiana[/name_f] (or [name_f]Christine[/name_f], [name_f]Christina[/name_f])
[name_f]Margot[/name_f] [name_f]Emmeline[/name_f]
[name_f]Margot[/name_f] [name_f]Evangeline[/name_f]
Of the suggestions before I really like [name_f]Margot[/name_f] [name_f]Cecelia[/name_f], [name_f]Margot[/name_f] [name_f]Isabelle[/name_f], and [name_f]Margot[/name_f] [name_f]Vivienne[/name_f]
But if you’re worried about too [name_m]French[/name_m] (though I don’t think it is) what about going with non-french spellings for the middle like [name_f]Margot[/name_f] [name_f]Isabel[/name_f] or [name_f]Margot[/name_f] [name_u]Vivian[/name_u]
[name_f]Margot[/name_f] is sweet!
[name_f]Margot[/name_f] [name_f]Genevieve[/name_f]
[name_f]Margot[/name_f] [name_f]Felicity[/name_f]
[name_f]Margot[/name_f] [name_f]Caroline[/name_f]
[name_f]Margot[/name_f] [name_f]Julia[/name_f]
[name_f]Margot[/name_f] [name_f]Susannah[/name_f]
I love the name [name_f]Margot[/name_f] [name_f]Isabelle[/name_f]-very pretty and not too [name_m]French[/name_m].
[name_f]Margot[/name_f] [name_f]Cressida[/name_f]
[name_f]Margot[/name_f] [name_f]Helene[/name_f]/[name_f]Helena[/name_f]
[name_f]Margot[/name_f] [name_f]Lorraine[/name_f]
[name_f]Margot[/name_f] [name_f]Louise[/name_f]/[name_f]Louisa[/name_f]
[name_f]Margot[/name_f] [name_f]Violette[/name_f]
This isn’t that helpful but [name_f]Margot[/name_f] is my [name_m]Boston[/name_m] Terrier’s name! I love her name! We call her Margotbae, Maregoo, Goony [name_u]Bird[/name_u], and [name_f]Margot[/name_f] [name_m]Wells[/name_m] [name_m]Fargo[/name_m]. Honestly, I was a little hesitant to name her [name_f]Margot[/name_f] because now I can’t use it on a human! It is a lovely name… 
My favorite middles for [name_f]Margot[/name_f] start with E (my current favorite is [name_f]Elizabeth[/name_f]). But here are some ideas for you:
[name_f]Margot[/name_f] [name_f]Felicity[/name_f]
[name_f]Margot[/name_f] [name_f]Helena[/name_f]
[name_f]Margot[/name_f] [name_f]Madeline[/name_f]
[name_f]Margot[/name_f] [name_f]Beatrice[/name_f]
[name_f]Margot[/name_f] [name_f]Jillian[/name_f]
[name_f]Margot[/name_f] [name_f]Caroline[/name_f]
[name_f]Margot[/name_f] [name_f]Lillian[/name_f]
[name_f]Margot[/name_f] [name_f]Felicity[/name_f]
[name_f]Margot[/name_f] [name_f]Genevieve[/name_f] is pretty too!
So far I think my favorites are [name_f]Margot[/name_f] [name_f]Lucia[/name_f], [name_f]Margot[/name_f] [name_f]Isabelle[/name_f] (or maybe [name_f]Isabel[/name_f]) and [name_f]Margot[/name_f] [name_f]Genevieve[/name_f].
[name_f]Margot[/name_f] [name_f]Leonora[/name_f]
[name_f]Margot[/name_f] [name_f]Stefanie[/name_f]
[name_f]Margot[/name_f] [name_f]Millicent[/name_f]
[name_f]Margot[/name_f] [name_f]Olivia[/name_f]
[name_f]Margot[/name_f] [name_f]Caroline[/name_f]
[name_f]Margot[/name_f] [name_f]Jillian[/name_f]
[name_f]Margot[/name_f] [name_f]Christina[/name_f]
[name_f]Margot[/name_f] [name_f]Ophelia[/name_f]
[name_f]Margot[/name_f] [name_f]Illiana[/name_f]
Best of luck!!
I love the name [name_f]Margot[/name_f].
As for middle names, I would choose something that means something to one or both of you as opposed to something random, unless you both like the same name.