As a functional machine, the contents of each gashapon need to entice visitors to spend a pound (or more) buying them. Anything too cheap, dull or badly made will defeat this purpose. Another issue is whether the items should appeal to existing 'universal' collections (stamps, coins etc) or invite people to start a set of completely absurd items (like plastic turds)? I found a philately (stamp) site which contains some very interesting points on the pleasures of collecting. The majority of these don't just apply to stamps, they are general hallmarks of what makes a successful collection. I'll be referring back to these as possible criteria for the gashapon content.
* They are aesthetically pleasing
* They are miniature works of art
* Desire to complete and arrange a set or collection
* National pride, we tend to collect stamps from our own country
* They have historical value, they show our Kings and Queens (or presidents)
* Intellectual stimulation
* Nostalgia, the good old days, memories of when I was young
* Hoarding and sorting instinct
* Sorting stamps to find a valuable one
* They might become valuable one day
* To avoid the wife
* Your Grandad gave you his collection
* I want to be the world expert on the 1d red
* To gloat over my collection
* You enjoy stamps - you have the stamp 'bug'
* You have an attention to detail
* To spend some time during a winter's evening
* You want to do philatelic research
* A financial investment