One problem with the fighting game bar idea: There's no guarantee that the player will understand what they're looking at. Hell, I've met fighting game fans, who play such games, yet still dont actually know how the damage works and what exactly the bar is representing. Doesnt help that not all such games even use that kind of system. And that's for people that actually play those.
It's one of those things that's often taken as a given by many gamers, like "How could anyone NOT know what this means?" despite the very high chance of many actually indeed not knowing what it means. It's exactly the type of UI problem that needs to be avoided here, particularly since mistakes related to regen/healing/shield values or whatever can pretty much end your mission in this game, and that happening because a part of the interface is funky would suck, for whoever is playing it. Roguelikes, in particular, generally do not use this kind of display for representing anything at all, which just worsens it. Alot of fans of this sort of game arent going to be the sort that'd play fighting games and have seen this, but instead will most likely be the sort that already plays roguelikes, or strategy games, or RPGs of multiple possible types, and so on.
Phew. This is a tough one. I guess it just depends on how obvious and intuitive you want to make things, then. The absolute simplest thing that everybody would understand and that works is simply having an active ability with a limited resource pool (exactly like stealth) that regenerates a certain portion of health per turn. Use it after a tough battle and if the area is clear no need to wait around. Use it before a tough battle to increase tankiness. Use it mid battle if you're worried about being one shotted. It's tactical, it mirrors the current behaviour of the stealth mechanic, and it works.
I am personally interested in this fighting game bar mechanic, but then it's nothing new to me. To others, as you say, maybe it would leave them confused as to how it works.
I don't think the idea of "push this button to heal the damage you took in the last shot" is much better, both mechanically (not a fan personally), and in terms of understanding. You can't really use it while under fire since giving up a turn while being shot is usually a bad idea. People are going to use it later, and they'll wonder why when they got blasted for 300 two rounds ago did they only recover 15 from the healing button?
The fighting game bar isn't just in fighting games: Dragons Crown uses something similar for boss fights. You may have to explain it to people, but they already have to discover how viruses work, how the AoE spread patterns on guns that aren't obvious bursts work (like the shotgun and it's odd pattern), and other things. So I don't think it's going to be a big deal. If the mechanic itself is sound, we can probably find a way to convey it in the UI.
Yeah, theoretically, it's not THAT hard to understand something of that nature, but.... like I said, most of the people that'd be into this would be the sort that arent likely to have played any games that use something like this.
You can also sorta look at it similar to how stealth originally worked; what you had there was sort of a "dual" UI element (I cant think of a better term as I"m not entirely awake), which had to show not only how much total stealth you had, but how much you could use in any one go. Essentially.... it was doing the same thing we're talking about for regen. And it confused the heck outta some players. And really, when you think about it, it actually wasnt very complicated whatsoever, but it WAS proof that something like that can still be confusing for some players. And if I recall correctly, tooltips and such DID explain it, yet it still confused.
For something like this to work best, it needs to be instantly recognizable, without much (if any) explanation or previous context necessary. Something like sentries, viruses, mines, or the current stealth implementation: you look at any of those, and there's a nice simple number there, you know "Ok, I have this much of that right now". There's no other information there to process (and thus confuse).
The whole thing isnt a particularly big deal, but I do still think it needs to be considered. The one thing that I have heard, every single time an Arcen game is released, is people being confused about something within the interface, or thinking something is not giving them enough info (even if it really is giving it to them just fine, which it usually is....).