Many of the plants we commonly grow are good candidates for experimanting with. Campanula should be pretty crossable. Any plant where you see a billion different cultivars on the market should be a good candidate for playing with.

One thing to keep in mind though with any of your experiments - if you are starting with hybrids, there will be a possibility that the plants you are trying to cross aren't fertile. Several hybrids on the market are sterile either through chance or through deliberate breeding. Also, some hybrids don't produce strong healthy seedlings - they may allow disease or bugs to gain a foothold in your garden. Inspect all these seedlings regularly and pull any that don't look so good, and you should be fine.

Buy go ahead and try - I often sow seeds of hybrids knowing that it's going to be a mixed bag of looks and success. Even if the new flower isn't as big/colourful/scented/whatever as the one it came from, it'll still be a pretty flower! And it'll be nicer in my eyes because it's 'mine' :-)

BP