Once upon a time a long time ago D&D was not owned by WoTC. 20 years ago in the dying days of TSR the company released the players option books. They were.
Skills and Powers
Combat and Tactics
High Level Campaigns
Spells and Magic.
And here they are from my personal collection.
For those of you wondering about modularity and subsystems for 5E this has been done before for D&D. Mike Mearls is a fan of 2E. In modern terms these books would let you add various subsystems to the game or optional rules. I will do a brief recap of the major points of each book.
Skills and Powers.
This is generally the most controversial book of the lot and good luck finding a grognard DM that will let you use it. This book contained things like point buy build your own races and classes and where points left over from an early progression could be carried forward to the next part. For example you could build a Elf with Drow bllod who gets to use some spell like abilities and left over points could be carried forward to the class section. If you want a Ranger who can't cast spells one could do that as well. The book also introduced things like basic kits (subclasses), variant ability score sub abilities, and a way to completely replace the 2E weapon/non weapon skill system and use character points. Several optional rules in this book made it to 3.0 and a form of the advantage/disadvantage mechanic from 5E is also in this book. Under the DM control this book is not to bad, under player control its a munchkins wet dream such as having a cleric who can heal, access to all wizard spells and can wear any armour for example. Or fighters wandering around with d12 hit dice and multiple weapon specialization.
Combat and Tactics.
This is perhaps the best Players Option book and the most useful in a somewhat normal 2E game. It more or less updated the Complete Fighter Handbooks and added things which were added to 3.0 such as grid based combat. This book is semi infamous for having critical hit tables in it where one can decapitate your opponent or crush their skull in. It also expanded on the weapon specialization rules, wrote weapons and armour allowed by era (Roman/dark ages, 17th century etc), variant materials, and also 4 pages of different weapons including lots of polearms. Also had things like catapults, trebuchets etc. This book would make for an easy update to 5E and one could put mass combat rules in it as well.
Spells and Magic
This book had a few things in it typical to later D&D splat books but it also errated and updated earlier material from the Tome of Magic and The Complete Wizards handbook. Around 20 odd wizard subclasses were included such as the 4 elemental wizards, Song, Shadow, Force mages and the Artificer. Alternative casting methods such as spell points, different power sources such as defiling, and things like possession were also here. Several of the new spells in this book also made it to the 3.0 PHB such as Cats Grace and Cure Moderate Wounds. Rules for high and low magic worlds were also included.
High Level Campaigns.
This was the Epic Level Handbook for 2E and probably the one least used by my players ever. I think we made it to level 19 once in AD&D and that was a monty haul game played when we were 15-18 years old. As the DM it had some interesting things to use in it. A prototype version of the flawed Epic Level magic system from 3.0 also originated in this book. The least useful of the lot of the books with 1-2 chapters useful for the DM. Unless your campaign is going over level 20 I suppose.
And that’s about it. These books are controversial among the grognard community but I actually like them as a DM as I would use them for a campaign. For example if one wanted to run Game of Thrones for D&D these books would be very useful. Letting players do what they want with them especially Skills and Powers is a very very bad idea as we actually tried this once and broke the hell out of the game. Between these books and things such as the Complete series, Historical Reference books and the 2E setting though 2E to date remains the most modular version of D&D ever produced. A lot of the modular ideas discussed in 5E are nothing new and 2E for example had a point buy build your own class system in the DMG in 1989. One could easily run some of these rules right out of the 2E books in plug them into 5E. Books like this also do not tend to bloat the game either as the rules were explicitly optional (DM may I) and there was very little traditional crunch in them mostly being new sub systems to add to the game or replace the core rules. The newspells in Spells and Magic being the main offender I suppose in terms of crunch.
Also thinking about the next introducing XYZ post. Thinking about making it Oriental Adventures for a blast from the past and what could be added or updated to 5E.
Yes Zardnaar, I own all of this series. I mostly used Combat and tactics, and High level campaigns in our AD&D 2e. games. We still play AD&D 1e, and 2e. today. This TSR era system never loses it's luster for me even after all these years. That said, I am enjoying playing 5e. right now also. I am not sorry I purchased any of my 5e books. I would play either game and enjoy myself. The thing is- what I like about 5e; is due to it's throwback feel at the table likening it to AD&D in many ways.
I have been a bit confused by your practice of mix and match systems for your own games as of late. Did you not find that any prior or current edition of D&D meshes with your preferred play style? For some reason I thought 3e was your go to game. I know you played C&C for a while as well. Did you like Castles and Crusades? I'd also ask what exactly are you looking for in "your D&D"?
I had these books. I was so excited to buy them and read them! 
I was a big fan of Rolemast 2nd ed back in the day and in some way this made me feel like D&D was taking an approach that would offer more options similar to Rolemaster......I guess thats where they got the name....."Players Options" LOL :D. unfortunately I think I remember allowing like....all of the options and it totaly broke my game. I made everyone revert everything back standard 2e but allowed them to keep all of there XP. In retrospect It was my own fault as I should have playtested it before I allowed all of the options, live and learn. But I loved the cover art and I remember them being quite cool. Thanks for the memories. (I still miss 2e sigh)
Spells & Magic also had, in my opnion, the finest magic item creation rules ever printed for D&D.
I am very picky about what I use from those systems. I liked Spells and Magic. The crit system was brutal and we abandoned it. I really lijed the Channeler and the different takes on spellcasters. There was nothing in the High Level campaigns that we found usable.
While I own them and dig them, I have never used them, they could really start a min-max nightmare, split ability scores...*shudders* But I have been perusing previous edition material for things to convert, will have a look at the PO books to find some gems.
Brightmantle wrote:
Brock_Landers wrote: