Tectonica
Tectonica is (well, would be) a game taking place in a fantasy pulpy setting, akin to the one in the Conan stories. The game would center around the city of, well, Tectonica. Tectonica is a free settlement in the middle of the desert, encompassing one of the only sources of water in miles, and is kept alive thanks to the caravans of merchants that regularly stop there to resupply and trade. Unfortunately for its inhabitants, Tectonica is also built on the strange and monolithic ruins of an ancient civilization far before the dawn of humanity. The city is surrounded by the ancient buildings, huge blocks of stone, most of which don't seem to have any opening whatsoever. Still, some do, and seem to connect with others in what seems to be an endless mazes, full of eldritch remnants of a civilization not of men. This has attracted to the city two other classes of people, both equally undesirable - adventurers and wizards. Thanks to those who come in the hopes of plundering the city of its pre-historic bounty, the place is far less safe than it could be. Carelessness ensures that awful things never meant to walk under the sun in this era regularly come out to haunt those living in the city. As a result, the place is very compartmentalized, with the richer living in relatively safety of what seem to be unbreachable walls of unknown construction, while the poor litter maze like passages in the city, more exposed to the eventual monsters and oddities that may roam those places, as well as the barely human mutations that live at the edge of society.
Game
At first glance, Tectonica would play like a text adventure, with inspiration taken from the Zork series (both the original three games and the enchanter triology), the Legend Entertainment games and the Quest for Glory games. The game would feature a graphical room view, similar to the ones in the early Legend games. This view could be interacted via mouse, allowing for the most obvious kinds of actions (such as leaving rooms, getting objects, opening/closing things). However, more specific commands need to be typed into the parser. The idea here is that there would be a host of different things different PCs can do in the game, but thanks to the parser, it requires the player to explore the gameworld, pay attention to everything he finds and try out new things to find them.
The basic gameplay is similar to that of Adventure games, but the way you interact with the world depends on what kind of attributes you have. I would use the word "puzzle", however I think it is important to stress that the game wouldn't have its gameplay built arouns a specific path the player needs to take like many adventure games have. For instance, if you need to go through a door, you might do it in several ways. You might steal the door's key, or find a way to lockpick it, or maybe cast a spell to open it. However, this game would go way further than the simple multiple puzzle approach in games like Quest for Glory. By having certain systems simulated, the player can explore opportunities created by them. For instance, an NPC schedule might mean someone might go through that door every day. If your PC can hide nearby and go through unnoticed, you would be in. Not because the designer created a specific check in the code to see if you did that, but because the schedule and sneaking systems interaction in such way. And the "puzzles" the game present are themselves more like interactive subsystems (like many of the puzzles in Maniac Mansion) rather than the puzzles in, say, Day of the Tentacle, where you are much more often required to just find the right item combination.
However, although the gameplay is based in adventure games, this game would be an RPG. You get to make a character, get to roll his attributes, possible mutations, choose a class, a profession and several skills. Depending on those, the game would play very differently. If you get the knock spell, then doors will be much less frustrating. If you learn to track, you would be able to avoid fights that would certainly kill you (besides, you know, being able to track). Being able to hide in shadows and move silently would open up all kinds of possibilities. Knowing how to properly use a shield would make you much better suited to fight human(oid)s. But against, say, rats, they aren't very well suited.
Combat
Combat in this game would be, up to a point, a tactical affair. However, it would be more like a logical puzzle than an optimization problem. That is, it isn't so much about reliably lowering the enemy's HP while keeping your own high. It is more about how you react in combat. If you approach those rats with strangely human hands holding your shield in front of you and your sword high, it is probably going to climb up your leg, possibly into your clothes or armor. Non magic shields are nigh useless against an ogre, unless you have really high strength. And of course, parrying their oversized clubs isn't possible unless you can match their strength, especially with a normal weapon. Skeletons are pretty hard to kill, unless you use a blunt weapon to their skulls.
Thus, the combat system depends a lot on how you approach your opponent, what stance you have, what weapons you are using and how you attack them. This is not a question of each of these things giving you bonuses or penalties for different actions, but also a question that different enemies are able to act differently. Keeping a parrying position with your sword is usually a good way to stay alive and even find an opening in your opponent's defenses. However, shoggoths like to suddenly project jaws from unexpected places into their victims, making this not of much use against them.
Your stats and skills are crucial in combat, however. For instance, dodging a charging enemy so he has his back to you is a very good technique against slower foes. But if your dexterity isn't up to the task, then you will need to find some other way to deal with them.
Magic
Magic in Tectonica is supposed to be mysterious, dangerous and, ultimately, wrong. Magic is a perversion of natural order, an affront to the very creation, a denial of the inherent dignity of everything and everyone. Magic has the potential to be very powerful, but it can sometimes backfire or even corrupt you (much like in the Dungeon Crawl Classics RPG). In order to use magic, your character has to not only learn the magic, but prepare it. This, I hope, will mesh well with all the puzzle themes of the game. For instance, a shapechange spell might have as its preparation the construction of a throne of skulls, with each skull used allowing you to take its previous owner's form. Thus, if you want to use something's (or someone's) form, you need to kill it without destroying its skull. You will need a way to actually build a throne out of skulls. You could, however, take someone's appearance and fool people into thinking you are that peson... if they don't know the guy died!
Ideally, the wrongness of magic would be relied in all of its aspects, that is, what it can accomplish, how it can go wrong, how it can corrupt you and what you need to do in order to use it. However, I don't want to simply be gratuitous here. First, I want this dark side of magic to actually touch the gameplay, or else you get something like Diablo, that has a very interesting "lore", which nevertheless doesn't matter one bit during the game. I also don't want to follow the same path as Carcosa (the RPG), or Lamentations of the Flame Princess, or even of FATAL. I mean, magic in LotFP is interesting (and with much more tasteful than FATAL), but in the end, magic there just evokes desperation. That whole game line is always about desperation only. A bit like Lovecraft's own stories (if a lot less dignified). I want something more like Robert E. Howard's stories, where at least some characters do have a chance of actually succeeding. By the way, I know this isn't always the case with Lovecraft, but he was much more likely to write this kind of story.