Minecarts that have to follow a track would be cool. Especially if you can go all roller coaster while building it.
This is why I want mine carts for mining.
To RIDE in Adventure mode!
And mine cart traps where you run over goblins or dump them in the mine cart and send them flying off a cliff/into lava.
Would make an awesome way to start a 'hermit' fort from a successful 200~ dwarf fort. You choose 1 dwarf, probably the newest immigrant who has no friends yet and separate him from the rest of the dwarves. Then you set up something to kill everyone in the fortress, like a connected series of collapses and monster releases or magma flooding (from an upper z-level, preferably with some sort of drainage so it just destroys the items but leaves the chambers) and build a get-away minecart line which leads to a safe spot outside (or even a safe chamber inside) the fortress. Stick the chosen survivor in the mine cart, set him rolling and then activate the collapses/magma and watch the fortress crumble around him as he zooms off to safety.
Would be an awesome ride!
And would inject challenge into the game, having to scrape off a living from whatever is left of the fortress and avoid whatever horrors you've uncaged.