This post relies on familiarity with the gameplay / affordances in Thief. You may want to read Dark Past or my write-up on "Assassins." You're also encouraged to read through Mr. Monahan's Design Reboot blog.
I've been watching Thief 4 coverage over the past months and I'm increasingly convinced that I'm going to hate it. So let's pretend, just for fun, that I'm making a Thief-like... What are the best qualities of Thief to preserve, and which parts of the sacred cow would I lop-off? Here's what I'm thinking:
KEEP: Level size. Thief levels vary from medium to extremely large. "Life of the Party" was a single mission where you had to traverse an entire cityscape of rooftops through two dozen buildings and then also infiltrate a huge skyscraper at the end. Sheer architectural scale is important to maintain a sense of...
KEEP: Infiltration and exfiltration. You have to get in, rob the place, and get out. There's always an outside and an inside, and it's great to feel that you're not supposed to be inside. But previous Thief games usually neglected exfiltration as redundant backtracking or worse -- something totally nonexistent because you've already incapacitated the entire guard garrison by the end of a mission. (My favorite Thief 1 mission, Assassins, is one of the few missions to really do something with exfiltration.)
ADD: Organic use of dynamic lighting. With dynamic lighting and shadows, you can do totally unimaginative setpieces where you have to hide in a guard's shadow or hide in the shadow of moving objects on a conveyor belt. But something else is going on here: with dynamic lighting, open doors and windows become light sources -- and if you can close the door or block the window, you've "extinguished" that light -- but if there's a ramp in front of that window, you can't put anything there to block it because it'll just slide down, etc. That possibility sounds much less horrible than a contrived puzzle that you designed to death.
ADD: Systemic response. In Thief, the level never fights back; it just waits to be "solved" into a barren landscape of corpses, extinguished torches, and unlocked doors. This feeling of territory and dominion is satisfying to new players, but this solved state is also very boring, to the extent that high-level Thief players voluntarily "ghost" levels -- never alerting any guards or leaving a trace, thus preserving the complexity of the level intact. Perhaps guards should revive / respawn somehow, and re-lock doors or re-light torches, to take-back territory from players. (But when they do so, they should be as slow and loud as possible?)
ADD: Procedurality. Thief, at its worst, heavily rewards memorization when you observe clusters of guard patrols to pinpoint their moments of weakness, and then promptly blackjacking them. I want to promote quick pattern recognition, improvisation, and brief situational awareness instead -- maybe a Hotline Miami approach? I think that might require mutating level layouts or patrol paths with varying degrees of procedural generation, even halfway through the level, so that the general structures are similar but the details are different. Don't bother supporting specific solutions or play-styles, let the player sort out the situation themselves...
ADD: Accessibility. Thief is notoriously unforgiving -- or at least it used to be. I think there is room for a sliding scale that lets less-hardcore players use Dishonored-style wall hacks and reading aids, but also rewards more experienced players with a wider possibility space and more complicated results if they forego the training wheels.
CUT: Permanent KOs. Ideally, there should be a tactical dilemma between trying to knockout a guard vs. not knocking-out a guard (e.g. just dodging the guard to steal some loot). In practice, the vast majority of players embark on relentless concussion rampages, which depopulates a level / severs connections between systems / makes the game boring -- so the optimal choice results in a increasingly worse game. I think something else should replace this dynamic.
CUT: Garrett. Okay, I get it, you like the character. That doesn't mean that character has to be in everything.
CUT: Anglo-steampunk. Okay, I get it, you like dashing inventors with charming steampowered contraptions in a Western early-industrial setting. If you want your Victorian pseudo-Gothic steampunk fix, you can now play any number of games or watch any number of action films. The world doesn't need another steampunk game set in Magic London.
CUT: Too many player tools. Why so many different arrows, mines, and bombs? Is it possible to combine one tool's function with another, and condense these tools intuitively? Are the gas weapons anything other than overpowered throwaways? The player inventory is bloated. Let's streamline the standard arsenal a bit.
Too bad it's all conjecture, and I'm definitely not working on a Thief-like and burdening myself with extra projects for no good reason. That would be foolish.