In my opinion, should have a feature that allows you to completely edit the shopkeeper shop, with any item you want and any price you want. This would help a lot for people that want to play a private server but doing something that only public server has, the NPC's selling specific things, such as evolution item shop, decoration shop, pokeballs shop and etc.
As far as I know, the only way to do this is changing the files or by programming the NPCs, so this is why I think this would be way easier :)