In the end, all trees bare some sort of "fruit" nut or cone, so to define rules, perhaps you would say that it must meet criteria like:
Fruit must be a focal point to displaying the tree
Directly edible for humans
Etc
Pines for example, many have seeds that are edible. But by no means are pine cone seeds a feature of displaying pine bonsai, so they would be disqualified. Juniper are edible, arguably the fruit are not really a feature, but the berries have to be processed to become Jin or tea, so that would disqualify it.
I'll argue here, largely from my culinary experience.
Juniper berries are frequently used as a sort of spice. I routinely use them in marinades, and I often add a couple to my pumpkin pies. So I would say a juniper is arguably viable in a fruiting tree contest, but may violate the spirit of the rules if not the letter.
Also, it's not unheard of to display pines or similar with cones when they happen. It's only rare because the things we do to trees in bonsai often interrupt fruit/seed production in various way. That might be stress on the tree, pruning away the tips of new growth where cones would normally develop, controlling nutrients to favor foliage over fruit, etc. Again, though, this might be seen to violate the spirit if not the letter of the contest.
Otherwise, I really like the idea of a fruit contest, but other issues with that have already been brought up. Another one is that different trees produce fruit at different times of year. If we do accept pines, many species produce comes one year, but they don't ripen until the next. So, generally, is the fruit required to be ripe at the time of judging, and how long do we accept final entries for judging.?
We might be able to limit it to trees with no sort of training every, but otherwise anything goes. This is essentially how the native tree challenge went. In that case we'd have lots of leeway for getting started on something that runs 5 years, but that would still make it a rather narrow timeframe for getting both display worthy entries AND fruiting.
We could also just make it as simple as a fruit management challenge: can you get any tree in a pot at all to bare fruit at the time of judging.
A variation is we could just go with an edible bonsai challenge, and allow things like thyme and rosemary, or whatnot. This, however, puts fruit on a whole other difficulty level, so judging will be difficult.
But then we could also just call it a bonchi contest, and all grow chilli peppers for 3 years. Capsicum annum often only lives 2 to 3 years, so styling, fruiting and longevity call all be part of it.
