Explorable compund of locked doors and deadly armed persons and monsters. Limited on-person inventory capacity shared between keys, weapons, healing items, and other equipment, which calls for deliberate selection and routing from storage locations. Never once feeling like less than a monster of a one-man army yourself, picking off squads of the enemy faction and grappling with beasts as you advance along your chosen path. Occasionally encountering roaming minibosses that require specific tools to actually put down and encourage deviating from your chosen path to flee.
Is this a solid, playable foundation? Any more thoughts on the matter?
This sounds like Overblood from PS1, a terrible game by the way.
Like I get what you are trying to convey in terms of gameplay, you want a furtive game where you spy on a place full of creatures and monsters that can easily thorn you apart, feeling pressured and claustrophobic.
However...
When you mix "explorable compund of locked doors" with "limited on-person inventory capacity", what you are going to give to the players are reasons to not explore it, like imagine having to choose between two weapons, some healing items and 6 different key cards, all because you found a new item, and you don't know what the game is going to give you next, a boss fight, a new corridor with items, a new key item??
Also, Limited on-person, that means that every character has its own inventory, right? If that is true, how are you going to separate key items for normal items in characters trades, what happens if a character dies, do the player lose the items? If a party member dies or is separated in a cutscene, and he is holding an item the player needs, then what happens? How the characters exchange items, they need to be next to it other, does the game pause when that happens, do they use a type of machine, a combination of in person and some machine??
"Never once feeling like less than a monster of a one-man army yourself, picking off squads of the enemy faction and grappling with beasts as you advance along your chosen path", what would be the difference between the monsters and the humans for them to not look like reskin of each other? How the battle between humans and monsters would differ? How would you balance their difference in a way that humans and monsters feels different, but at the same time, they feel like equally terrifying threats of the same game? Would They work completely separate or sometimes in conjunction?
"roaming minibosses that require specific tools" what would happen if the player doesn't have the specific tool at the time of the battle, after all, the inventory is very limited, so the player would either have a non-disposable useless item taking space of useful items or some kind of item respawn, that could break the game economy easily.
What you could do is...
Limited types of item, like 20 max healthy items, 10 gun types, etc. or a Resident Evil inventory, but with different inventory types, like "I could use this big gun, but it would take the space of 2 medium guns" and "I don't have more space for medicine items" both at the same time, with key items like cards, essential machines, etc. being in a limitless stricter space, basically a mix of Pokémon and Resident Evil inventories
just don't the every character has its own inventory, it would only results in frustration, is it realistic? But if real life was not frustrating, video games would not exist, and games are a fantasy abstraction of reality, sometimes it is better to sacrifice realism in exchange for gameplay.
About the "roaming minibosses that require specific tools", do it like the megamen games, where these tools are useful in other scenarios and, if you use a certain item or item combination the battle is easier, but if choose to do it otherwise, it is okay, in that scenario, not only the specific tools would have other functions, but it would also not soft lock the player nor limit his options, also you could implement a challenge that is basically fighting with the boss without the use of the typical weapon.
The "advance along your chosen path.", I had the impression you were being inspired by Shadow the Hedgehog way of changing endings, do not do that, this type of game works wonderfully being a more narrative driven with you choosing between your initial mission and being a traitor, an idea of doing that is besides the dialogues, some actions, but also, give the information translucid for the player, like if you do a good thing, the moral compass goes whiter and a high sound effect plays, otherwise the moral compass goes darker and a more tense sound effect plays, to give the players the notion of what path they are taking in a Pavlov way, but dialog tree also works as it is the standard.
The "never once feeling like less than a monster of a one-man army yourself, picking off squads of the enemy faction and grappling with beasts", this one was just some questions for you to think about it, there are a lot of games they do that in both bad and good ways, just search and try to see the style more akin to what are you trying to achieve, understand what went wrong and why, but also why that worked and why.
Also, never forget about balancing the enemies, the weapons and the player what system would you use, level design is essential specially to teach the players how the game works without using a tiresome tutorial, a redo gameplay would be nice, where you can just skip a cutscene until the moral choice pops up to get the other endings without needing to read it all over again, also plan your endings to feel like different endings, quality over quantity.
What you posted sounds like the beginning of an interesting game concept, if crafted well, it would be an Excel game, good luck.
Btw, if I said something wrong, some of the ideas I gave were bad or just want to add more things and give feedback, please quote it, I would love to read others input on that matter.
