Some good ideas, here are a few other cool things that could be done with minecarts...
Minecart engine (Replaces "Minecart with furnace"):
- When crafting, the furnace has to go at the right or left of the cart and will be placed with the 'front' of the engine at that side when placed on the rails. The engine would be the same height as a fence post.
- Starting the engine would work with a click as per a button/leaver or it could start with a direct redstone current (Not power rail or activator rail). It would stop with the same click, running out of track or hitting a block. (If the block is removed it would go again)
- Starting the engine will make the rail under it act like an activated powered rail with slight forward momentum.
- It knows how fast it is travelling and will activate the rail it's over as required to reach 'top speed' (so going up hills will use more or if the cart is going over a powered rail, not use any)
- Each 'rail' it passes over that it has to activate to keep a constant speed uses one unit of fuel per cart in the train.
- The Engine has one 'cell' of storage for fuel as per the current furnace; The fuel will be used as per any furnace smelting, but could also burn TNT for give additional speed boosts when used.
- A setting in the engine could allow for 'hard breaking' (stop as if it it hits a non-active power rail) or 'soft breaking' (just decelerate as a normal cart).
- Placing any cart on the rail behind an engine will start a chain with subsequent carts being able to be added. (No leads required; just set minecarts onto a rail adjoining one that already has a minecart on it and they automatically couple.) Holding one minecart type and click on another already placed to swap them (if minecart containers are empty). Minecarts placed in front of the engine will not couple and be hit by the engine.
- An engine cart added to a chain of other carts will 'end' the train and automatically face the opposite direction. Starting the engine on one end will de-activate the other
- Aesthetically it could produce smoke (per the new 'campfire') when activated. Could burn fireworks for speed and sparkly smoke.
- Can stand on the engine as it is moving, but acts like any block; can be knocked off.
- There would be an internal 'slot' in the engine for the front of it:
- a light source (eg a torch) it would turn on a front light.
- a sword would increase the damage when it hits a mob
- a pick would mine X when it hit a block at the end (depending on the speed of the cart)
- a slime ball would shunt into other carts without breaking them (and would stop) It would also not damage mobs
- an observer block would automatically start and stop the engine if a player entered/exited the train.
- a note block would warn if a mob is on the line (and make them get off the line)
- A basic cart without a chest in it could store as much as a hopper and the graphic would fill up as entities were placed in it. These would all be ejected with an activator rail or by breaking the cart. Detector rails would change output depending on how full the cart is. You can only get into an empty cart. Minecarts hit by an Engine act like they were 'punched' by a player and will break.
- A hopper cart placed behind an engine (or another hopper cart) will connect when it's placed and dispense into the engine. Hoppers in carts could also be updated so that they collect rogue items from blocks adjacent to them like they do from the blocks above them.
- A regular hopper could connect into an adjacent rail so that it could dispense into any stationary engine/chest/hopper cart.
- The dispensing direction of a dropper cart, would be determined by the dropper's position next to the cart in the crafting table (no front or rear dispensing); Sides and above would activate the dropper with an activator rail while moving and like a normal one if stationary. If the dispenser is facing down, once activated by an activator rail it would lay stored track on the adjacent block in-front of it until it runs out or it is activated again.
- A slime cart could allow block placement on top (and a half-block on the sides). These would move with the cart in the same way that a slime block does on a piston. Each block counts as an additional 'cart' for fuel consumption. Two slime carts could be bridged and count as part of a 'train' even though there would be no minecarts between them.
- A Note-block cart could make it's sound when it goes over an activation rail.
- A fence cart could make a cage that would work like a hopper for mobs (same storage and collection). (Perhaps use iron/nether fences for hostile mobs. Only collect non-agro mobs.) This would be the only way to get mobs into carts; running into them with a cart would now cause damage. Could still interact with mobs in cages (creepers blow up, skellys shoot, villagers trade, animals feed but could not breed).
- It would be really useful if the TNT cart was made useful specifically for creating tunnels specifically for rails/minecarts; if the explosion was directed in a cone forward from the direction of travel. (Or even if the TNT explosion was set x in-front of the minecart.)
With the new aquatic update it would be also be cool if rails were able to be placed and used under water:
- I would make the carts slow down twice as quick while under water.
- Perhaps a new mine cart with a glass block to make a "bubble cart" where you can breath underwater if you are in it.
- Mobs might drown if taken through water, some food items might "spoil" or automatically be ejected to float free when a cart containing them enters the water.
- Empty carts could fill with water and have to be emptied/broken (creating a water block) before they could be used again.
- Bubble streams and vortexes could have a chance to separate carts and 'break' trains.
It would be cool if a mine-cart could be turned into some sort of cable-car: if mine-tracks could be laid on the underside of blocks and carts would follow under them. Perhaps you would need to craft some sort of hanging bracket that would accept a mine-cart 'bucket' (This would be amazing in the nether)
If you ran a rail track into a village, then that village would have a chance to spawn a new villager/trade type that included all the useful railroad stuff.
Putting a compass in a cart (And/Or the new cartography block?) would add a 'rail network' onto any relevant map for the unbroken length of track the cart is on. (including tracks hidden from a normal 'satalite' view and areas not explored on the map).
Putting a sea-lantern or glowstone (or other light block) in a cart would just produce a light source that could be moved by rail.
Crafting a cart with a dye and a slime ball would tint the exterior cart's colour. With an Engine it would colour the boiler section.
Signs and item frames could be attached to the side of carts (and move with them)
Name tag could be used with an Engine to place a name plaque on the side of it.
I'm also thinking that an Engine model should cover 2 blocks and let you ride in the second one like a cart. Don't know how it would handle corners, but it would look cooler.
I also think it would be cool aesthetically if we had some of the old 2-man, flat-bed, see-saw, style cart that moved when you pumped the handle up and down... no idea what function it could provide, but it would look cool (esp. in the bad lands)