Maybe working for a minecart made out of a NetherFurnace? That would be playing with the Furnace Minecart gui, but changing it that it work like the NetherFurnace.

Example :
In the Nether, you get 25% of the power to go foward, enough to go straight pushing one cart, or itself on a hill.
Adding some coal bring another 25%, that would burn for power (in the overworld don't have the nether bonus, so its the same as in the nether without burning source).
Placing some lava in it would be a 100% boost, which let it climb hills while pushing one cart with a player, or two empty/chest/etc cart.
That would do a furnace minecart that is more usefull in the nether then in the overworld, pushing one extra cart on a flat surface, or one with a player and one with a chest.
The NetherCoal can be a 75%, to reach lava power of overworld in nether.
Thoses are random values for example, would still be to decide.

EDIT : The furnace minecart don't have a gui, you simply use the fuel on the cart and it go when it get push.