Make character count change color according to threshold
One nice feature Aegisub had was how the character count for lines/subtitles changed color the closer it reached the preset character threshold. The farthest you were from the threshold, the colder the colors are, and the closest, the warmest.
It's been a while since I used it, but IIRC the colors would get fully red when a line reached its maximum, and breaking it into a second line would get the colors colder, reaching red once again when the two-line subtitle reached its maximum.