unnecessary addition; gilding the lily
A yojijukugo meaning to ruin something by adding an unnecessary extra touch, from a Chinese fable about drawing a snake and adding legs. Used to criticize superfluous improvements.
His suggestion is an unnecessary addition.
This explanation is gilding the lily and actually makes it harder to understand.
蛇足 is a shorter, more common synonym meaning 'redundancy' or 'superfluity', derived from the same story. 画蛇添足 is the full four-character idiom and is more emphatic.
From the Chinese fable 'Drawing a Snake and Adding Legs' (画蛇添足), where a man lost a contest by adding legs to his snake drawing, symbolizing unnecessary additions that spoil the original.