The simplest way I can explain what this is would be to desribe a Car. Cars accelerate and decelarate. I took a crack at it and couldnt get it to work right. What I was thinking was to be able to call a script like "event.speed_ramp(new_speed = n, dist)" and have dist default to 1 but be optional to gradually change speed over several steps.
I've found that using non whole number speeds for the player causes some issues in tilesets to appear to "tear" so it wouldn't be used for the player so much. The intent would basically be for vehicles like boats, wagons, or cars. Not sure if anything like this has been written already.