I'm going to make this short and sweet, since a lot of suggestions in the past have perhaps been a little too long.

Increase the speed of minecarts

Why?

• It makes them actually worth the investment, otherwise you might as well just use horses and boats

How?

• There are plenty of plugins that work fine already like this one m8 ^{Hint: You don't have to use that one, plus it's not exactly complicated to make a plugin for this purpose. It's one goddamn variable.}

Are there any problems?

• At speeds notably higher than the default speed, minecarts turning, or going uphill can get derailed, or bounce back and change direction ^{Hint: Make the portions of rail in which you want the carts to go super fast be flat and straight}

Interesting? Yes? No? Possibly? Thoughts?