This is a pretty far off thing, but at some point i think it would be great if we could have shop/guest items turned into objects to make them available for customization. For things like custom food items, custom souvenirs, custom trash, etc. It would allow for a lot more depth with custom stalls since presently you're limited to just the selection of items that are hard coded into the game right now.

For food, i think you can get away with just having them reference the guest sprites for food items already in the game to keep things simple on the peep animations side of things. As many food items already in the game do this (eg: cookie & pretzels share the same guest sprites) so you should be able to get away with just using the animations already in the game for most custom food items. Metadata for them could consist of what guest sprites to use, what trash item to turn into when it's consumed if any. Consumption time, etc.

For custom souvenirs, they could just be simple ones that only appear in the inventory like toys but i think there's potential for allowing you to create custom accessories like balloons, umbrellas, and hats. RCT3 for 8000 example would let you have a balloon stall that would sell crocodile shaped balloons, so you could have stuff like that in OpenRCT2 with these. This would probably entail having the balloon entities also get their sprites from these objects so they keep the same appearance when let go.