[name_m]Alexander[/name_m] and [name_f]Philippa[/name_f] would be so darling and elegantly vintage!
[name_m]Alexander[/name_m] and [name_f]Harriet[/name_f] would be my second choice.
To me [name_f]Abigail[/name_f] feels too modern with the timeless [name_m]Alexander[/name_m]. It’s kind of the same with Felicity; even though it’s a very old name, the first thing that pops into my mind is 90’s [name_f]Keri[/name_f] [name_m]Russel[/name_m].
[name_f]Isabel[/name_f] and its many variations feel a bit overused at the moment but [name_m]Alexander[/name_m] and [name_f]Isabel[/name_f] would go together beautifully.
Good luck!