I have 2cents to chuck in to the pillow idea fountain.
Not knowing the context of your sofa, your taste for kitsch and preferred colors , it it kinda tough to give you opinions. Not that I am deterred from doing so!
Please keep us posted on how your room comes along and congratulations on the bigger pad!
Consider using some other shapes - barrel , rectangle to break up the massiveness of sofa and existing pillows.
Maybe go for a moire or tone-on-tone solid.I keep thinking reds, purples, turquoises with the beautiful chocolate.
Different textures like silk, satin or velvet .
Perhaps a more modern-type floral? Marimekko-ish?
I always like Chinese Indian and Thai, all of which should soften the look but add nice counterpoint to the modernisty pillows you have as well as tying them to the more British Colonial sofa thing.
maybe someting animally , like an embroidered elephant or even faux-leopard spots.
If you have a Chinatown area , I'd head there for pillow covers - nice vivid jaquards 9or fabrics to make yr own.
I love Pearl River in NY but you can shop online or at least window shop.www.pearlriver.com
Pier One has good bunches of "Asian" designs but I can't bear to go in them because of the overwhelming, toxic stench of scented candlles, potpourri and such.
Still, the website is safer!www.pier1.com
Crate and Barrel has always had great stuff, great quality and often nicce markdowns:
Ditto Target. Each store has different stock so it is worth hitting more than poone in your area if poss.
They usually have really good pillows, particularly in their 'World Bazaar" area.
This place had really great stuff on the website - and inexpensive!! http://www.worldmarket.com/
And finally: Ikea
Wow - i need to go do something about my sofa cushions too!