That’s a change from the last few times I posted it, where I said you only spend HP if you succeed. I could see it either way. Sharpshooter gives -5 to hit for +10 damage, and that’s automatic if you hit. So you could do variants. Spend first and lose them right away, but for eveyr 5 HP spent you do 3d4 instead of 2d4. Declare first, but if you miss you don’t lose 5 HP but if you hit you only do 2d4.
This is a repeat of an idea I put into a few other posts, such as my comment about Marcus the Paladin’s fighting ability as well as the Barbarian. I want to be able to refer/link to it independently.
The gist of it is: going toe to toe with a melee expert should be a bad idea. You can spend your own HP to inflict more damage on a foe when you hit them in melee.
Going face-to-face with Sir Cuisinart should really be scary. Granted, with plate and a shield, AC 20 (more with magic) is nothing to sneeze at, so they’re harder to hit. But the per-turn damage ability of fighters is really second to a lot of characters that are second-rankers.
I was thinking that it might be interesting to allow something like a fighter to spend his own actual Hit Points for more damage. Say, 2d4 to a blow for every 5 HP you spend. You probably don’t have to limit it other than not being able to spend below 10 HP plus your CON bonus times your level – those are mostly physical toughness, not skill or grit or honed combat instinct. So Marcus could exchange HP for extra damage dice as long as he’s above 28 HP (3 for CON x 6th level + 10 HP).
If you did want to limit it, perhaps you can only do it a number of times equal to, say, half your level plus some attribute score. Not STR, because that double-dips. CON might work, but that seems odd since those are physical HP, and this bonus damage is from skill. DEX might be fun, since honestly for heavy fighters there’s not much reason to do DEX. Well, unless you’re an archer. WIS might be a good one, as it’s the ability to notice an opening (tied to Perception). CHA makes no sense, nor does INT. Half level plus WIS? That would allow Marcus to do this five times, trading 25 HP for 10d4 damage to his foes.
I’d make you select whether or not to spend HP before you roll to hit, because I would double these damage dice on a critical hit – that means, though, that you probably lose the HP if you miss. You exerted yourself to strike the foe, but it didn’t work.
So Marcus might announce that he’s spending 5 HP. On a hit he would do 1d8+2d4 + 5 (8-21 HP), but on a critical hit the dice would double: 2d8+4d4+5, for 11-37 HP range.
This would leverage a fighter’s higher HP for something other than a damage sink and pincushion, and make it quite risky to stand face-to-face with a fighter type. For that reason (and since Hunter’s Mark and Colossus Slayer stack already for archers, and give plenty of extra damage from a distance where the archer can’t get hit back) I’d make it melee only.
Hrm. That might be a good Feat, actually. Or even a Fighting Style? Class feature for combatives, and Feat for those that might want it?
Obviously it would need playtesting.