I've been tossing around a comeback mechanic for combat-based games that I thought it would be interesting to see other thoughts on. The way it would be balanced would have to depend significantly on the nature of the rest of the game, so if you do have thoughts please mention the context you imagine them in!
I've dubbed the mechanic battle rage, and at its core it's pretty simple: you chose to begin expending your maximum hitpoints in exchange for a combat advantage. This means that the loser is encouraged to use rage, as they're not losing anything by it, but the winner is discouraged. This is why it's important to balance it contextually - if it were in, say, a 5v5 MOBA situation, using rage would affect your ability to win future fights. If it's in a 1v1 single-round scenario like a classic fighting game, it would have to be managed pretty carefully so as not to make it overbearing.
My first thought was to draw inspiration from various time-manipulation/action games and have rage slow down everything except you. This'd be fine in a purely PvE setting, but in a PvP scenario it would lead to the only response to rage being to counter-rage (a la atium in the Mistborn series, or your choice of other time dilation effect). I feel that that excessively favours the person on the lower health total, as unless the speed advantage is small, it essentially provides them with a way of reducing their opponent's health to their own over the span of a few seconds.
The other option in my mind that seems reasonably original (or at least not super heavily used) would be depleting your maximum health in order to replenish your current health, in the manner of an adrenaline-fuelled pain tolerance. This option is particularly interesting if there are other ways of healing available (probably out of combat), since then there are stakes to winning without raging - rage's advantage can be reproduced without the side-effects, as long as you can get out of combat (hopefully by winning - I don't know that I like running being encouraged). At the same time, it's possible to tap rage briefly in order to gain a small advantage at lower cost.
The last option is to have rage simply buff offensive and/or defensive abilities. I don't like it, since it feels completely bland and uninspired, but it is fairly easy to tweak the balance on.
All of these I have in mind as being a constant buff while you drain your maximum hitpoints (except, in a sense, the heal - the gained hitpoints are persistent). That way, you get one shot at making it back into the game, and when your hitpoint deficit runs out you have to burn real health to keep your advantage.
This has a slightly awkward consequence that both sides have access to the comeback mechanic, so if you drop to 25% health while your opponent is unharmed and then use rage to level the playing field, you're left at 25%/25% health while your opponent is at 25%/100% with the option of burning rage to close it out.
I'm not sure if I'm sold, but one possible option is to have rage only useable when you're at a health deficit. That way, there's no way for the winner to abuse rage to cement a lead. The other way to prevent the problem seems far kludgier, and would just be to have attacks made while raging damage hitpoints and maximum hitpoints in equal measure.
Thoughts? I find it an interesting problem, but it has so many contextual constraints...