Well, I went and made a retroclone. Here's a link for the PDF: link
Basically, I found myself annoyed with certain aspects of 5th edition Dungeons & Dragons, and I wanted a simpler system to run with a friend who's a gaming novice, and those two situations happened to coincide with my stumbling across the whole idea of OSR/retroclones. I checked out the rules for a few systems (Swords & Wizardry, Labyrinth Lord, Lamentations of the Flame Princess, OSRIC, For Gold & Glory, Beyond the Wall, and Dungeon Crawl Classics), and while they all had cool stuff that I liked, they all also had some degree of bits and pieces that I didn't. Rather than using one of them and stacking house rules on top, I figured I might as well grab the SRD and do my own thing with it.
I used 2nd edition Advanced Dungeons & Dragons as the basis, but I tried to think through the reasoning for having everything that I did instead of copying it dogmatically. I rather like what I came up with, although I'm still early enough in the process of playing it that I'm sure some problems will come up.
So, what makes this different from the products that are out there already?
As mentioned, it uses 2E AD&D as the main skeleton, which is something that nobody aside from For Gold & Glory did. Personally, stupid number of spells notwithstanding, I think 2E is a nice compromise between the depth of mechanics of 1E AD&D and the clean simplicity of B/X, so I thought that making a smoother version of it would be a mostly-novel effort.
The core set of rolls is split between using d20 vs. Target Number for attack rolls and saving throws, using 2d10 roll-under for ability/skill checks, and using 2d6 vs. Target Number for specialty checks (like learning new Wizard spells or checking morale). This keeps a chaotic/unpredictable feel to combat and magic and a sense of reliable expectations of competence for ability checks. Specialty checks being on 2d6 does likewise while increasing the impact of various modifiers.
Saving throws are condensed into 3 categories (the classic "fortitude, reflex, will" of 3rd edition), each adjusted by the worst modifier from two stats (strength/constitution, dexterity/intelligence, wisdom/charisma, respectively). This keeps some distinction between classes based on saving throw categories for fairly little complexity and provides an incentive to avoid dumping stats (if you're using a stat generation method that even allows for that).
Ranged attacks have a simple and smooth penalty in place of stepped range increments. This was something that I'd tinkered with in various ways for a long time, but the real breakthrough came from reading this blog post on Delta's D&D Hotspot. I added a little more to it (namely diversification of maximum effective ranges for certain weapons and an additional consideration for "maximum firing range" instead of just maximum effective range), but Delta deserves the bulk of the credit for that idea.
My own idea for handling grappling/tripping attacks. It's basically a mixture of 2nd edition, 5th edition, and my own thoughts.
Mass combat system inspired by Matt Colville. Again, he deserves the bulk of the credit for it, though I did make some changes and merged it to some degree with the siege combat rules from 2E AD&D.
Conversion formulae for mapping creature stats from other systems into this one.
A fair amount of useful DM advice. I doubt it's enough to actually count as a good guide for a complete newcomer in and of itself, but it's a far sight better than what I've seen in other OSR books. Hell, I'd even say it's better than a lot of the "advice" in the official Dungeon Master Guides from TSR/WotC for any editions that I've played. I'm not saying it's a better product overall, but seriously, the actual DMGs for 1E AD&D, 2E AD&D, and 5E D&D are pretty bad about helping someone get a handle on what to actually do.
Now, admittedly, it's not quite complete yet. I still have to fill in a lot of spell details and bestiary entries, put in the tables for treasure generation (I have an idea of how I want to do it, but I need to experiment some to make sure I'm getting sensible results), put in details for magic items, and write up the appendix for adventure design (which is mostly about putting my notes into a form that could be useful for someone else, especially for the two sample adventures). For all of that stuff, the best I can suggest in the meantime is to supplement with the free PDF of For Gold & Glory.
Finally, to be perfectly honest, I don't have any real expectations that other people will use this. I'm quite aware of how little traffic this blog gets. Still, in the event that anyone does use it, I'd love to hear any feedback.