I'm sorry Kovarex but I have to agree with the other dissenters here - I don't think this feels very good from what I have felt the "spirit of the game" to be. Ultimately you get to define what that is, though, since it's your game, we're just playing it.
To me, Factorio is about how well you can put together a logical set of steps to accomplish a task, not about whether you have paid enough time and materials to the RNG gods to give you the same stuff that doesn't look different but functions differently. That feels more like a loot box.
I do agree that I would like to see the dynamic between horizontal and vertical scaling continue past where it stops right now, but I don't think the way to do that is by changing how we value almost every item in the game.
What I DO Like
I love the idea of extending the horizontal / vertical scaling portion of the game.
I like the idea of there being more "realism" in a factory - after all, machines make mistakes, wear out, need maintenance, have problems... etc.
I like the idea of a 4th module that does something sideways to the main game.
Why I Think This Isn't Resonating with People More
It feels like we've just taken one of the most engaging aspects of the game, the tiered upgrade system, and trivialized it down to some RNG modules that amount to colored dots, visually, and that doesn't feel very satisfying.
Consider - we could implement the Furnace tiers and have it be functionally identical (if we disregard size), with your Quality system. A regular furnace would be nothing, a Steel Uncommon, and an Electric Rare. We could just put colored dots on the original Furnace art and ship it, call it a day, job done.
But we don't do that. We have different recipes and different art for the tiers of furnaces. Why?
Because it's much more satisfying, as a player, to have researched and worked for an upgrade to a critical component of your base, and see those shiny new furnaces replace the old ones that were less efficient and more polluting. If we had a series of stone furnaces with dots on them, that is much less rewarding to play with and have worked for.
I don't think Factorio has a whole lot of RNG problems to work out - the Uranium processing getting U-235 is the only example I can think of off the top of my head, and that *feels* more justified because it's temporary until you set up your Namesake Kovarex Enrichment process which produces it reliably.
Most Importantly...
Please remember that if you design a game to have a "better" or "perfect" tier to achieve, no matter how well you balance around it, players will want to achieve that and de-value everything else.
Anything else will feel like varying degrees of Losing.
Players will also act like it on the forums and in their interactions with each other, as a result of this design. We will eventually see messages and posts from players denigrating each others' creations because they aren't 'Legendary' tier. "Sure that works great, but why isn't it
Legendary?" is a pretty shitty thing to hear, and honestly, to feel internally while you are base building.
The fact that it exists at all is a thumb on the scale of the player psyche - it does not matter whether you've cleverly designed it to be "optional" - when it is the proposed level of integrated in to the game, it's a BIG deal, not just some random side recipe a person could have fun with if they wanted.
How I Would Do It
I would reverse the design - I would put the RNG in the
processes instead of the
items.
I would borrow a page from Space Exploration's book and make all assemblers (or building producers) of lower tiers have a pain-in-the-ass chance of producing Scrap instead of the item they intended. I would leave the player's manual crafting alone. I would make this scrap somewhat useful and recyclable, but it would ultimately be a tax on the resources being gathered.
You could implement the RNG mechanic in the recycling of the scrap. When a chunk is processed, depending on what it came from, maybe it produces an iron plate, or a copper wire, or both. Maybe better tiers of recyclers can sometimes produce rare metals they collect over the thousands of piece of scrap they process.
I would expand the vertical / horizontal upgrade gameplay with new recipes. The higher tiers of buildings (Assembler 1 -> 2 -> 3 etc) require better ingredients, and therefore produce their items more reliably, the same way the game is designed now. Steel gears instead of Iron gears. Advanced circuits instead of basic circuits. Incremental improvement, same as real life.
This *feels* a lot more realistic and more in what I believe to be is the spirit of the game - a scrappy made-for-a-purpose factory to accomplish a task that has an almost never ending list of sub tasks; just like a large programming project IRL.
A little bit of eventually reliably solved frustration is ok at first. It gives players something to work towards, as you already know from your Bot balancing post where Cliff Explosives were mentioned to be moving to a higher tier of tech, but it has to be more rewarding and reliable to get out of than what you're implementing here with Quality Modules.
I love Factorio, the same as everyone else here on the forums does. I wouldn't be spending the time and energy to think about and write out posts like this if I didn't, so I hope you don't come away with the impression that I'm just disparaging your art. I'm not. The game is fantastic, and even if this were to be implemented as proposed, it wouldn't be the end of the game for me or I'm sure anyone, but we can always improve.