I have spent too much time thinking about first person shooters for someone who has not played one in years. Here's a sketch of a system encompassing scifi power tools (and, correspondingly, guns). This was written with a (forthcoming? who knows) mashup of Orbiter's Local 519 and Maze Rats in mind, working title: HULK RATS.
In the general course of events, characters have Stamina which is depleted by strenous activity or injury. Every time they become injured, roll (an appropriate type of roll) under your stamina or take Critical Damage. However, when a crisis that needs to be resolved moment-to-moment breaks out, they can rely on a Rush of strength and agility to protect them instead.
Once a character becomes aware of danger, the player may spend points of their stamina up to your level (if your game has variable hit dice per class, two-thirds level for clerics/thieves and one-third level for mages [rounded up]) and roll that many dice. The total is your Rush, which is used for several things:
A player may only roll for a Rush when their character becomes aware of a new source of danger. An encumbered character can't roll up a Rush.
If they have a chance to rest after a crisis is resolved, characters may exchange Rush for Stamina at a rate of three to one. If there is no opportunity to rest, or if there are excess points of Rush after exchanging the rest for Stamina, they are lost.
Listen, there are a lot of different interpretations of HP. This is another one, clearly. Probably someone's thought of it before. A lot of different ideas are coming together to form this: hit dice as a resource to replenish hit point pools (sort of reversed), hit dice being rolled at the start of an encounter, Into the Odd-style critical damage rolls, and (weirdly enough) an example consequence for failing a 4e-style Skill Challenge being the loss of a healing surge (out of combat) or damage (in combat).
Oh, and the rule about spending Rush to take actions while you're being menaced is taken from the Pressure mechanic from OLOG, an extremely cool replacement for Attacks of Opportunity which is (as far as I know) only described in the parenthetical note to the Arsenal ability of the Marine class in this post.
One thing I like about this is that the consequences of surprise/backstabbing sort of comes for free. I also think the Electric Bastionland rule about burdened characters having zero HP fits in very naturally. Not sure what else to say, I just kind of like it.
