If you could somehow blow a really cool looking CO2 diffuser.......that would sell like crazy. Maybe a spiral looking thing that would diffuse the bubbles into the column farely well. I bet a lot of planted tank guys would go for that! All the ones being made now look like crack pipes. Thats all I have.....let me sleep on that question.......maybe more ideas tomorrow?

diffuser would sell, they are hard to come by. I think there is only 1 other glass diffuser on the market. you could also make wierd rock-like structures. Maybe add sand to the glass to add a dirty look and it will also make it gritty.

I'd take a look at some of the diffusers on the market, there are a few. The best advantage you could create in marketing your own is selling at a lower or equal price. Some of your already established possible competitors such as ''ADA'' put a lot of research into their product. When people go out and and spend a good deal of money on such a small piece of glass they do it because the design is sound and gets the most efficiency out of its purpose (ie, the shape acts to dissolve as much CO2 into the water column as possible). The biggest advantage to a glass diffuser is how inconspicuous it is. People don't generally like ''equipment'' to be seen. In most cases it would be all but engulfed by plants anyway, but..

Here's some common ones:
http://www.drsfostersmith.com/product/p ... catid=9915
http://www.adana.co.jp/_e_product/22_index.html

As far as like artificial looking plants and seaweed or whatnot, I'm sure you could create a market somewhere keeping in mind that anything really fragile may not last long. The tank glass/water distorts vision so much I find myself bumping things all the time inside the aquarium. Bumping a thin glass piece of ''seaweed'' into my tank wall would probably not bode well. O.o

Best wishes in whatever you come up with.