Rockets + Minecarts = Fast

If the minecart is standing still, the rocket will behave as normal.

If the minecart is in motion, then the angle you're facing determines what happens.

If your viewpoint is at an angle less than 90 degrees from the direction of travel, the rocket will cause the minecart to speed up. (I'd prefer it accelerate all the way to Elytra flight speeds, but I doubt the game makers will go for that.)

If your viewpoint is at an angle 90 degrees or greater off the direction of travel, the rocket will cause the minecart to slow down, stop, or jerk the opposite direction depending on the initial forward speed.

This will give rockets a pre-End use for transportation purposes without making rail travel too competitive with Elytra, because the same rockets can be used for both.

Nor does this damage the "specialness" of Elytra, because Elytra is still the only method of air travel that exists in the game.

Maybe it could give out sparks if it goes too fast