I do not think it is very helpful to think in terms of what can be done with any given game. I think it is more helpful to think in terms of what is expected and how difficult it is break from those expectations. Does the system help you? Does it stay out of your way, but is not particularly helpful? Does it actively work against your interests? When I speak of system here I am including all the attended social and cultural pressures that go along with playing any particular game, not merely the mechanisms. I mean we do not need formalized mechanics and principles to roleplay. They can help us do things which are not particularly natural for us to do, but could lead to more overall enjoyment. I think it is a mistake to assume that the lack of formalized system means no system is in place. We simply default to the natural constraints we put upon ourselves. In indie circles this is known as the Lumpley Principle.
We have a tendency not to see the ways we are used to doing things as constraints, because they do not feel constraining to us. While few mainstream games speak to their principles there is a body of principles and social expectations that go along with the design of most mainstream games that I feel are deeply constraining for my interests. This includes the expectation that a GM will either engage in lengthy world building or use someone else's world building, that a GM will prepare adventures that put designs on how players should interact with them or else use someone else's adventures, that a GM should engage situations to enable spotlight balancing, and that a GM should advocate for a particular narrative. It also includes expectations that a player should enjoy exploration of the fictional world and a GM's story for its own sake, decide how to approach everything as a unified group, puzzle out what they should be doing at any moment, not engage the mechanisms too deeply, not engage with parts of an adventure that are meant to highlight another player's character, and not interrogate the fiction too deeply. These are social expectations involved in playing most mainstream games that I find deeply constraining and that tend to cut against my interests.
Yes to world building and using somebody else's world building.
No to prepare adventures that put designs on how players should interact with them, and rarely somebody else's adventures, almost never as written.
I can't stand spotlight balancing, nor do I advocate any narrative. Add in that I'm not particularly concerned about game balance in the same way others are.
I don't have any specific expectations that a player should enjoy exploration of the world, although I hope they do. How the world is explored is really up to the players, and I can adjust to their preferred style. What I do hope they'll enjoy more is the exploration of their character.
Working as a unified group makes things easier, but I'm also happy to run multiple parallel campaigns with one, two, or more characters at a time, and bring them together if/when appropriate. Nor do I think that there are parts of the story meant to highlight anybody (no spotlighting) so they are free to interact with what they'd like.
Oh, yeah, I play D&D. That's about as mainstream as it gets.
However, my approach is one that's developed over 35+ years of DMing, with home-brew A&D having set the stage so to speak. So the development of the world and what I expect of the rules is built from that. The 5e rules are just simpler and more elegant - with modifications.
Having grown up with the game during a period where home-brew was published monthly in Dragon magazine, I consider my approach "mainline" in the sense that the game has always been (to me) malleable and flexible, the intent to make it work for you.
I also acknowledge that somebody who starts playing now will expect relatively simple rules with fast advancement, and be able to pick up a "campaign" (AP) to play a character more or less from start to finish. If you started in 4e, then you'll have different expectations from the game than if you started in 3e. I think 4e was more prescriptive about the play style. You pretty much had to use a battle mat, whether you wanted to or not, for example. I think 3/3.5e required quite a bit more investment in learning the rules, only because there were so many variables. AD&D (at least up until the
Complete series of books came out, and 5e, are much simpler from the player's perspective, for different reasons.
In other words, D&D is as mainstream as you choose to make it. But I get what you're saying, because when I run public campaigns, I feel very restricted in what I can do (or not do) as a DM. If I choose to limit races or classes, it's often a big deal, for example. What I think it really is, is the expectations that come with D&D. Although those expectations can vary widely, there are a number of things (like the ones you mention) that tend to be relatively common.
My solution? Well, one of them is that I love new players. I love helping them put together a character, and letting them loose in the world. No need to read through the rule-book - just tell me what your character does. The world is similar to ours in terms of physics, and you can really do, or attempt anything you'd like. We'll tell you how to figure out whether you succeed or not.
Of course, this sounds an awful lot like AD&D - the DM worries about the rules, you can focus on your character.
