It would have to be one damn good attack for it to replace speed AND power.
In order to get the most out of a character, you have to use all his or her abilities to the fullest.
If this character lacks speed and power, and only has one really good attack, you can bet your life on it that this one attack will be spammed to death, and that way, nobody will be happy. Also, by adding another attack instead of upping Trunks' speed and/or power, the Team would contradict itself, because they always say Trunks is a Melee character, not an energy attack user.
Trunks should have have a speed that is only slightly below Krillin's (otherwise he would become unbalanced) and his Transed speed should be close to Cell's. He should have an added damage ratio for his sword attacks, and keep the 20 percent insta-kill.
I think that would optimally enhance and balance Trunks.