I know this idea has been partially suggested already but mine is a little different and I wanted to really flesh it out.
Once crafted a wagon would be a surface that you can build on (to an extent) and move with horses. This would allow players to make different kinds of wagons like a Mobile Base, Mob/Player Transporter, War Wagons, Grain Trucks, and anything else they can think of.
Firstly the wheel would need to be invented. An easy way would be in a crafting table like in picture 1 in the attachment.
Once this is done players would craft a simple wagon. This could be either in a crafting table or built and usable once all pieces are in place (similar to an Iron Golem). I need input to which way is preferred. If in a crafting table it could look like picture 2 in attachment. It would be a total size of 3 blocks tall, 5ish blocks wide, and 5-6 blocks long. But the platform area would only be 4 blocks high, 3 blocks wide, and 3 blocks long. Picture 3 is a top and side view.
With this platform the possibilities for the amount of uses for wagons are endless. For example you could have a mobile base where you can park it for a minute and have your crafting table, anvil, furnace, chests, etc. all available to you without having to set them up. Picture 4 shows this.
Another use is as a mob or player transport by placing fences. This would be great to move villagers to a closer village or animals from across the map to your base. Picture 5.
A third example could be a war wagon where dispensers are placed on the edges and pressure plates in the middle to launch fire charges and arrows at mobs and players. Picture 6.
This list could go on with all the possibilities of a simple movable building platform. Different building options could include using a hopper in the middle to make a ‘Grain Semi Truck’ to allow for easy unloading by dumping out the bottom. Picture 7
A final variation would be a double wagon which would be accomplished by combining 2 wagons together on a crafting table to make a double wagon for more space and more carrying capacity and would require 2 horses instead of one. The end building space would be a 4 blocks high, 4 blocks wide, and 4-5 blocks long with 4 wheels instead of 2. Picture 8.
Also some items (but not blocks) can be placed on the sides of the wagons. For example you could place torches or ladders on the sides of the wagon but cannot place anything that you can't walk through essentially.
I don’t know how this would interact with slime blocks if at all. Also I need input on the speed at which wagons would travel. I think a double wagon should move slower than a single and both should move faster on a gravel or stone road than on dirt or other earth types. You might need to craft a harness or something to put in place of the saddle in order to attach the horse to the wagon. I'm open to suggestions.