I dont think Finish Buster needs to be changed at all. Once you get Finish Buster, you can just sit there in the air and spam it at people who are charging up their ki. I have done it many times when I start to get bord in the game and it will kill 90% of the time due to how fast it moves and how much damage it does.
Trunks is a melee character anyways. He already has the sword and can gib with it. That is why his ki attacks arnt that strong. If you were to make is ki attacks stronger, then he would be the best character due to the low ki needed for transformation, the sword gibs, and his ki attacks.