I'm primarily hoping for alternate magic modules -- and I don't mean spell points, I mean complete, fleshed out magic systems.
Love it or hate it, the Wheel of Time novels present a complex and internally consistent magic system with a lot of depth.
Brandon Sanderson's Mistborn novels present another (in fact, THREE).
If 5e included channeling rules (not the disastrous second-rate hack job they did in the 3e WoT book, but real honest-to-Jordan channeling rules), I would buy 5e, no questions asked. If they included allomancy, feruchemy, and hemolurgy rules, I would personally fly out to WotC headquarters and hug every member of the design team until security kicked me out.
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Spoilering my thoughts on modularity for length. Sorry, I just tend to get verbose about these things.
[sblock]
So, something that's been a part of the game since at least 2e shouldn't be handled?
And yet D&D has done it
twice, 2e's Options, and Mutants and Masterminds (if you'll allow the stretch.) Some of 2e's Players Option were pretty kludgey, but Mutants and Masterminds seems to be rolling along on all cylinders.
So its popular...but wrong?
I'm not making any implications about
how they go about achieving anything in any of the modules I mentioned. Nonetheless, if Gestalt characters can be a mess and still popular, I think that goes to my point. In fact, I was holding them in reserve as an example.
Monster races, point-based advancement, and gestalt are all examples of popular subsystems that have been around for multiple editions. They are also examples of subsystems that are clunkier, more vague, and less coherent than they could have been because they were added on as an afterthought.
Certainly, someone might
want to change the feel of the game by including certain subsystems; many people use gestalt because they want to power up their PCs, or use condition tracks and wounds to make PCs more fragile, or the like. That isn't modular design. Modular design implies compatibility, not drastic change; it implies cohesiveness and planning, not one-off ideas and post-hoc changes.
As you mentioned, all pre-4e editions have included an Unearthed Arcana book full of interesting subsystems to tweak the game. And that's exactly what UA should be, and where those game-changing subsystems should be. I'm not saying you shouldn't have V/W hit points or other things in the game at all, but there's a big difference between modular design and UA subsystems.
UA subsystems are minimal implementations, almost outlines, with plenty of "If you use this subsystem, you should..." and "This could be broken if you..." advice. They don't take any other parts of the game into account, they tend to require DM intervention to fully incorporate, and they often change the feel of the game quite a bit. DMs who use those subsystems are probably comfortable with the "Advanced" part of the rules, as you put it.
Modular design, however, while it too only touches a small part of the mechanics, is self-contained. Rather than giving you an outline of changes and leaving it at that, a module should handle all interactions internally. You shouldn't need any "If you use this module, you should..." advice, because a module, if done correctly, handles all of that transparently. A module's effects should be predictable and they shouldn't screw anything else up.
Saying the the core must be designed to easily include/use them is just sensible. I certainly have no objection to that. I don't even object to that being a design goal/criteria. I just don't think that a few difficulties are sufficient reason to forbid a module from ever seeing print. Especially since DMs still houserule, etc. anyway, IME.
To make it more clear, let's use psionics as an example.
3.5 psionics is a module. Inserting it into a game requires no changes to existing material; it provides classes, skills, powers, items, etc. of its own. Psionics material follows the same formatting, balance guidelines, etc. that "normal" material does. You can run a game including psionics and not have the game or game assumptions change drastically, and if you want to run a game without psionics you can ignore its existence entirely.
1e psionics is not a module. Going without psionics means removing psionic abilities from existing creatures, which means you've altered their difficulty and changed what they can do. Psionics material is not interchangeable with other material, and its balance is vastly different from standard classes. If you run a game with psionics, particularly if you have a psionicist in the party, it's quite different than if you don't, and if you want to run a game without psionics you need to do the heavy lifting yourself.
3e Vancian casting is not a module that can be replaced by psionics. While it is roughly on a par with psionics in terms of power, balance, breadth of capabilities, etc., so much material relies on effects from Vancian magic that aren't in psionics that it would be a major task to change things. Without Vancian magic, game assumptions change quite a bit, even if overall power doesn't. If you want to run a game without Vancian magic, you're on your own.
That's the difference between including these alternate subsystems in a UA-type book, which I completely support, and "Just throw it in a module, we'll both get what we want," which is irresponsible design. Saying to make it a module just stops the conversation. It assumes that both versions of the subsystem in question will be equally integrated into the game, that they'll both get the same amount of support, that they'll both be equally easy to use, and so forth.
People have been using "Use a module!" to say that people can play 3e-style and 4e-style characters in the same game, that people who like and hate warlords will be happy with the game, that people who like and hate healing surges will be happy with the game. You can't. That will never be possible, because the differences of assumptions and balance levls in the systems are too vast to simply modularize everything and have it all work out. You can
approximate characters of both styles, but only through DM intervention and UA-style add-ons.
Now, yes, if by "modules" everyone means "UA-style add-on subsystems that will be tenuously balanced, have unexpected side effects, won't be balanced, and won't get much screen time, then yes, you can easily modularize anything you want. But that's not modular design and that's not what either WotC or forum-goers have been meaning.[/sblock]
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On the original topic, more modules!
--First of all, I second Minigiant's Contacts and Guilds ideas; D&D hasn't done enough realms play since 1e and Birthright, and I'd love to see it come back.
--A lifepath system, i.e. a way to "organically" generate characters by saying that they have so-and-so background and so-and-so adventures and getting stats, feat picks, and such from that. This wouldn't involve any new mechanics, but rather would provide random generation and/or a fluff-to-mechanics converter to help you quickly generate characters.
--Honor and Reputation. Similar to contacts, both of these would give you favor and such with NPCs, but they would also provide mechanics for determining how well-known you are, could serve as a substitute for alignment, and similar.
--Aspects, similar to FATE. Plot control mechanics related to aspects of a character's personality and outlook, a framework for roleplaying XP rewards, and the like.
) . I hope this post makes my position clear(er), and bear no hard feelings to those who disagree as everyone seems to be legit in their concerns.