I think if we get right down to it an MMO is just a precursor to an alterate (virtual) reality. Meaning we'd all want to see one be more like "The Matrix" (not the MMO The Matrix obvs.)
And when I've been immersed in a game it's the pacing, seeming random encounters, and twists that make me involved and interested.
The mechanics of the game just have to exist transparently, and just not interfere with the world.
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That's why I like Skyrim, because mechanically you level up the things you do, and don't level up things you don't do. And you'd want the world to react mostly by how you react, but often times there would be times where you couldn't continue and need the help of others.
Basically you'd want it a bit more organic in some respects. Groups forming on their own accord rather that for loot or for repetition.
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I would probably get rid of world chat, or make it either linkshell or another technology. When voice becomes common enough; HD voice; proximity based where it's actually part of the environment. Shout and you'll be spotted, whisper and you'll stay hidden.
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Mostly, a lot of the other parts of immersion are almost here. There's the Occulus Rift, Haptic Gloves/Haptic suit and a 360 treadmill/or the like, and you're set. And I think this will pretty much available as a whole in the next 10-20 years.
If this is the case then 1. People would only play sparingly; because it would be a workout and 2. The mechanics would be more in line with your physical attributes and skill than computer dictated stats. Although those could be there too.
Imagine, going up to a wizard school and basically taking a class on how to cast a certain spell. What words to recite, what motions to go through, taking a test on it. Then saying and making the motion of the spell. You would admittedly look pretty dumb (like saying Fus Do Rah) but this time it's not clunky. But most of all it would be fun. And this is executed by good writing, pacing and the like.
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Realistically though, in the current gen. I'd rather see a slower calculated game that could be soloable. Where stats and skills can matter, but it's about making all of the available content fun. Replacing boring goals and benchmarks with activities that are fun in themselves.
You could focus on the minutia like in my above VR example and create an awesome game, and that's what the Final Fantasy series had done in the past. They are considered mini-games. And these sorts of things happen to be the best part of games like Skyrim as well.
Variety, different encounters, novel encounters are what keep things fresh, not reiterations of the same thing.
I've always like the meta-mini games where you would go into another world in game. Or sneak around an enemy camp, dressed as the enemy. Or breaking the 4th wall to poke fun, or breaking the 4th wall to show off a certain mechanic, like in Eternal Darkness. The MGS series also did very well with that.
Admittedly the reason why these sorts of things worked is because they are used sparingly and done well, but I don't think in current games they are used enough. And although this sort of novelty shouldn't be the crux of a gaming experience, they should IMO always exist in addition to other solid "vanilla" content.
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So yeah, sorry for not be more concrete and kinda in the future with all of this but these are things that I'd definitely pay $15 a month for.