Ethan Joshua-always liked Ethan, sounds strong and goes rather nicely with Olivia!
Samuel Aidan or Aidan Samuel- which flows best-It's a pity Aidan and all the other -aden names have been taken into the trendy realm, I love Aidan, but I'm not into overly popular names! I think both flow nice, Samuel Aidan would get my vote because it would be a bit less popular!
Max William-great names and sound nice with Olivia!
Caden Rhys-don't like the -aden names(I have a nephew named Brayden, it's just becoming really common!)
Jack Michael-oooooh,,maybe my favorite! How about Jack Samuel? Jack William?
Ian Michael-I think there is a comedian or actor with this name, you might want to google it, His name is Ian Michael Black I think!
Declan Joseph-good, sounds a bit gothic or something, not sure...don't like the "flow" of it with Olivia!
Out of the above I think Samuel Aidan gets my vote! Sounds really nice with Olivia Sydney, and it mimics her name in length and style without being "matchy"!
A few other combo suggestions:
Samuel Owen (my tacky side is showing! I reversed your DD's initials!, It sounds darling IMO!)
Caitlin Paris-So nice that you have used the real spelling, but it doesn't "flow" with Olivia IMHO. Paris is a name I can't stand, it will also date her name!
Bethany Paige-very pretty, doesn't "flow" as well as a few others do, with Olivia!
Leah Caitlin-LOVE Leah, my DD is Leah! Not sure if this flows with Olivia either!
Darcie Rose-very pretty name, one of the few truly unisex names that I like on a girl, but still, Olivia is very girly and feminine, I think you need to match that "feel" with your next child!
Jacey Ella-too unisex to go with Olivia
Sophia Paige-very pretty, Sophia "goes" well with Olivia too!
Megan Elisa-very pretty, will never seem "dated" or trendy!
Morgan Alyssa-ok...Morgan is a bit to "boy" for me
Mia Juliette-LOVE, prefer Juliet, much more classic! Great with Olivia!
Ashleigh Rose-I only like it spelled Ashley, it looks trendy and pretensious with this spelling IMHO. I actually prefer Ashley for a boy too, but it's pretty for a girl.
Out of the above I think Mia Juliet and Sophia Paige are the best fit with Olivia Sydney!
If I can offer a few other combos...