I feel the functional products are easier to market than the non-functional ones... the design based marketing follows trends, and those trends are guided by other marketing campaigns, aka, the biggest/hottest games at the moment (and that's constantly shifting too). The most secure are probably the classic / iconic ones, or appeal to big brand's fan base - Xbox, Playstation, Nintendo... but the bigger brands require licensing to market their designs / logos, I think.
...or... you go for color and abstract shape patterns that you make available for customizing, or allow the buyer to upload their own designs, but this last option seems expensive.