I think they should add "iron scaffolding", a non-solid block that rails, torches, redstone, ladders, and other utilities could be placed on , but would not themselves have a collision box. This would allow for minecarts going places that mobs and players could not.

Personally though, I think the entire rail system should be redone. The only reason it uses gold is because gold is useless otherwise, and someone suggested it to Jeb a long time ago through twitter.

I think they should remake the system sort of like it is now, with a few fundamental differences. For one, there needs to be better minecart physics along with linkable minecarts. "Furnace Minecarts" get a bit more advanced in crafting, and become more powerful. Instead of "powered rails" pushing normal carts, activator rails power "redstone carts", a high-cost but powerful and fast cart.

Both the upgraded furnace minecart and redstone cart would be equal power and would not allow for a rider. A "control cart" would not only allow controlling the speed and direction of a cart train it was linked to, but also would allow control what was pushing it-meaning you could use the furnace cart to push when there were no activator rails, or just the redstone minecart when there were.

I think they should make the rail system more useful as well. Not only should they be faster and easier to make, I would really enjoy it if you could use it to transfer resources (logs, sand, gravel). And no, I don't mean like a chest minecart, but something that has the same inventory of a double chest, but only for 1 material, like if you put logs in it, it can only transport logs until they are removed.

I don't really see any other kinds of rails. . . The only kinds I can come up with is something Mojang'll never add

Well, it'll be awhile until Mojang decides to add it. Probably 5+ updates.

There's actually a mod that adds those rails. It's called Rail Craft.