I am thinkin to create this booklet for my use where are only the basic d20 rules (retro) including both ascending and descending AC (for maximum compability). There would be no classes, spells, items, monsters or anything else. Those would be sourcebooks or taken from a game I run that time.
The point is to create small booklet with all the rules and easy to add own houserules. So every time I take another OSR game I can use my own rules booklet and just take the fluff and material (those spells, classes and whatnot).
I think the rules would be based on a hybrid of Labyrinth Lord and LoTFP, because LL is clear dealing with descending AC and LotFP is clear with ascending AC.
The base of these rules would be from BLUEHOLME (Holmes edition clone) because it's super clear and simple and it would be easy to add all those additional rules. Also BLUEHOLME is a small book, so the core of this universal retro system would be fast to create.
It would be also easy to add my house rules there. Just write rules BTB but add little boxes of alternative rules. That way I can easily see how the rules are BTB and the other options I invent or gather all around the internet. If I find something superior I can replace official rules altogether (although I think I keep the old one just in case and reference).
Also when I read "yet another retro-clone" rules most of it is the same basic mechanics, some things are better but some things are better in another game. So it would be painless to just add the parts to my game what I like as optional rules. That way my booklet could have several different optional rules for the same thing to choose from.
This would be kinda my personal rules Cyclopedia.
When I look at OSR games I find that actual rules don't take that much space. Character creation (without classes) and core mechanics (adventuring, combat, task resolution) take only a small portion when classes, spells, items, monsters etc. take the biggest part of the page count. How much are the core rules in pages? 20 maximum? Own additions and house rules don't add that much.
The process would be:
1. Take rules material from BLUEHOLME
2. Add and modify rules from LL
3. Add and modify rules from LoTFP
4. Add and modify house-rules when necessary
Also if there are rules variants I can easily do this:
Labyrinth Lord roll d20 and check the table
LoTFP roll d20 over AC
House-rule open fortune cookie and improvise
So basically when I need a rule I just find it from the booklet and choose what suits best for the game I am running. One book three set of rules for (almost) everything?
Also this core retro rules booklet would be faithful for its sources and my house-rules would only be grey boxes.
Currently I am running to different systems and I got house-rules literally everywhere (G+, blog, notepad, in my head, napkins) so it would be super convenient to put all this information in one place. Also it wouldn't matter what game I run when all the rules are in the same booklet. Only thing I need from different games is fluff rules and material (those classes, spells, mutations, items, monsters etc.).
So basically it doesn't matter do I run Mutant Future or LotFP, the rules can be found for these slightly different games from the same place. Only thing where I need the actual game's rule book is the setting mechanics (again classes, spells etc.).
It would work like GURPS. When you run GURPS (I use my rules booklet) the core book has the rules. All the additional material, genre, setting etc. I want to run is in GURPS sourcebooks (in my case Labyrinth Lord, Forgotten Realms Campaign Setting, LotFP, Mutant Future, Mutants&Masterminds etc.).
Also if I like feats I could easily add a section how feats work and use D&D3 list for available feats. Only make the rules simplier.
Also own setting material would be easy to work with. I already have core rules written down, so I can just write what I want. I can write a booklet about Wild West, take my booklet to run it and choose whether I use LotFP or LL for it (or house rules).
Also creating core classes would be easy for fantasy. Write class fluff and take rules to work with LL and LotFP. For scifi, wild west, post-apo, 20's gangsters I just need to rename and re-skin those classes.