TL;DR
Belts are fun. Rails are way cool. But there is nothing between... Enter the minicart!What's a minicart ?
A minicart is a small vehicle that would serve a unique role in the distribution of goods through a factory. Currently we have belts, rails, and logistics robots to deliver goods, but we don't have a type of roadway system. Minicarts could fill this gap. Factories could be built around and connected by a system of paths, much like road ways. The Cart would function similar to a train, with auto pathfinding and stations, and be governed by lights, stop signs, etc... The player could manually drive around on the cart, set them up to deliver goods, etc... They would have a much lower speed than a train, a smaller inventory, and essentially would operate on a smaller scale, however, they are not just mini trains.The mini cart can travel any direction, make tighter turns, and be governed by road signs.
Why ?
First, creating a system of timed lights, linked intersections, and four stops, and dealing with mincart congestion on the roadways would be novel in factorio. Factorio is all about logistics and solving problems. The belts offer a unique challenge, as do the railways, but managing traffic on a raod system would offer new opportunities to link factories in creative and novel ways, as well as present new and unique problems that don't exist in the belt or rail system.Minicarts would be available before trains and help bridge the logistics gap between belt and rail while replacing neither. It would give the player a better way to travel within the factory.
Creating a good rail network is all about flow. Creating a good belt system is all about balancing supply + demand. Creating a good mincart system would be all about sync and timing. In a imaginary setup, you would create a mini city where specific factories produce lesser used goods. The minicart system helps distribute those goods (lube drums, batteries, sulfer, etc) between adjacent factories . The rail systme is still responsible for delivering goods en mass and over great distances. The belts are still used to feed through factories, but minicarts link the two.
If nothing else, creating roads and managing traffic would be a blast and fit perfectly within factorio.
Details?
Roads/paths would be 1x1 and allow a minicart (also 1x1) to travel along it. Minicarts are multi-directional and can make 90 turns, reverse, etc. They would be constructed using an engine, 2 steal, 10 cogs, and 3 green chips. Their inventory space would only be a few (3-5) slots, They would require refueling. A second tier of cart would be made using electric engines and require paths with a built in recharge wire (like trolleys). Signs, such as yield, stop, oneway, and lights would govern the flow of traffic. Minicarts would have lower top speeds but faster acceleration than trains. One important feature of the minicart is a bumper. No need to worry about about minicarts stacking up at a light.Besides needing fuel, all minicarts would need lube to continue to function. They also require a new item to function at peak efficiency - an air filter - which must be replaced periodically and cleaned. Soilded filters can be cleaned at an assembler using water. A robust minicart network would require maintenance/refueling stations, dock stops for loading and unloading goods, and possibly recharge stations for electric minicarts. It could be possible to create a factory with no belts using minicarts, just as one might use logistic robots, espect that micarts would be more fun than logistic robots >.> As a side idea - Another network, called the transportation network could also be added, allowing minicarts to function automatically and return to a home dock when tasks run out. The contents of a loading and unloading stations would be part of the network, when goods conditions are met, the mini carts attempt to visit a nearby loading station, retrieve goods, and deliver them to a unloading station where goods are running out. Personally, I would like the manual minicart (where you set it up with stops like a train) but the real fun would be in solving congestion and creating a good light system to govern the carts, so I wouldn't care either way)
In the end, minicarts would add another valuable logistics layer to factorio. ald also perhaps be "easy" to implement as they could use some of the rail path-finding code.
. It's a shame cars (and wagons) can only be placed manually. 
