Peregrine and Lysander I like best of all. They don't strike me as plain at all, even next two the wonderful names you've already given your children. The only strike against Peregrine is that it so close to Persephone, compared to Wolfgang.
Mattias I love, but it will get mispronounced in the US. Anders is great but not with Oliver.
So... Lysander gets my vote. You could also try Leander.
Your two kids have excellent names. From your list my favorite is definitely Lysander (or what about Evander or Leander?), and I think it would fit well with your sibset. Peregrine would also be a nice mix with your sibset and would definitely be my second choice. To me, Zephaniah has too similar of a sound quality to Persephone (the Zeph and seph being such prominent syllables in each name, sound way too similar) to make it usable. Mattias and Anders are both nice names, but sound a little boring next to Wolfgang and Persephone. Jameson is the only one from you list I'm not fond of.
Here's a few other suggestions (sorry for any repeats):
My favourite from your original list is Peregrine Oliver- so gorgeous, like your other kids' names! I also love Lysander but I don't think it flows very well with Oliver in the middle. I saw Peregrine Lysander once and always though that was a beautiful combo.
I've got to agree with the Ulysses suggestion - just perfect with your other children's gorgeous names!
Wolfgang Felix 'Wolfie'
Persephone Mila 'Posey'
Ulysses Oliver 'Uly/Uley'
MAttias Oliver stands out to me and i think it is wonderful with your sibset
I like Lysander with your subset because you could nn. Him Leo for Lysander Oliver. I think that goes well with brother "Wolfie." otherwise I love Jamison and Zephaniah.
Peregrine Oliver - lovely and the nn Perry would be gorgeous
Zephaniah Oliver- again, beautiful - nn Zeph very cool
Jameson Oliver- I like this but I think it is a bit too bland next to your other children's names (but a great name in itself apart from being a whisky brand)
Mattias Oliver- love Mattias , this is on my guilty pleasures list (not really SOs style)
Lysander Oliver- One of my favourite ever names , again on my guilty pleasures list- this name makes be go all goosepimpley and has the very wearable nn Zander
Anders Oliver- ok - again not as standout as your other two , but a nice name on its own
My favourite would have to be Lysander Oliver (oooh very Shakespearian - maybe that's why I like it :D)
third - Zephaniah
edited : Wow , and have just seen the suggestion of Ulysses - that is pretty awesome too!
You've been using this site for a while but your join date was yesterday? I smell a troll.
I'm not sure that's fair, I used the site for name ideas for a long time before I made a profile and posted anything.