I understand the confusion, the strike-through didn't transfer like the bold and italic. I corrected the OP
Here's the intent:
On your turn, as you make an attack roll, you can use your bonus action to enable the "power attack" feature until your next round. You are not bound to use it on all (or any of) your attacks, but that would be the only way to use the -5/+10 on a reaction attack.
The cleave feature is free however, with a limit of once per turn
If you make the "power attack" a bonus action, you're going to completely nullify certain sub-classes. Currently I have a level 8 Samurai in a campaign, and she has the Defense style and has Great Weapon Mastery feat.
The Fighting Spirit component of Samurai requires you to use a bonus action to activate the advantage and temp HP components. I normally only use the -5/+10 component when I have advantage. However, if I want to use the -5/+10 component, I'd not be able to use Fighting Spirit. As it is, when I use Fighting Spirit, I don't get to attack another foe if I kill an opponent (or get another attack on a critical), as I've already used my bonus action.
What is proposed would seriously weaken the Samurai subclass. Other than flavor, why wouldn't I wish to go to Battlemaster, where I'd be able to augment my -5/+10 with one of the Battlemaster's maneuvers?
As I mentioned, the Samurai already is somewhat limited with Fighting Spirit with GWM. I wouldn't want to nix my character any more. She's still fun to play this way...with the suggested changes, I wouldn't have anything "special".
A well played high level assassin with the damage reducing element for defense already can go insane on damage, and can be as good as a fighter (I haven't analyzed the high level ranges yet - but the highest level I've ever reached on another character in two years of play is Level 12). Why do we wish to weaken the fighter moreso? Dexterity is such an important skill for for all characters and especially assassins, they are already only down one point of AC over fighters (17 with studded leather, 18 for fighters in plate). What does that leave fighters if you take away their abilities to damage? The assassin in our group does have fewer HPS than my fighter, but the assassin (often but not always) doesn't need extra HPs because uncanny dodge is so great versus the big bad foes.
In short, leave the fighters something that makes them great. They don't get high level spells or super nova strike capabilities. Just let them hit pretty hard (but not smite or assassin hard) when they use the tools they have (maneuvers, fighting spirit, familiars positioning themselves, etc.).
If you really want to see something crazy when maxed out, look at a ranger with the crossbow feat and a hand crossbow. My fighter just sits back in amazement when the ranger goes to town!
Right now Clerics, Bards, Wizards, Sorcerers, Fighters, Paladins, Rangers, Rogues, Barbarians, and Monks all have elements which make them super fun to play. Don't take away their toys! I've not seen enough Druids to give a fair assessment of how fun they are to play. The one class which I've found no fun to play is the Warlock. They need so many short rests to do anything exciting, it isn't fun at all. Taking away GWM from a fighter would make so many fighter builds unfun like the Warlock. Let the big guys and girls swing a mighty stick and have some fun too! Don't be a hater!