Campanula generally needs more sun. Classics are impatiens, coleus, begonias (both tuberous and fibrous but also consider angel wings which can be brought in before frost).
A perennial you've missed out on is rodgersia (big bronzy leaves and white or pink bloom a bit like astilbe - wich is another thought along with solomon's seal). Few annuals will self sow in your climate - coleus might if you have good snow cover.
Winter aconite and snow drops are also good in shade.

Impatients are great for a shady area. I like the dwarf kinds best...they don't get leggy. Torina are great also. I've gotten the 'Clown' series and they do really well with just 2 or 3 hrs of sun. There are other kinds of torina, but I've only tried the 'clown' series because they've done so well for me. I've never had these reseed for me, but they're very easy to get seed from them.
Good luck!

Kat

browelia and cosmos are others you might want to try.

browelia and coleus are others you might want to try.

Mimulus Monkey Flower
Myosotis sylvatica Wood Forget-Me-Not
Violets