I barely ever use the forums on this site, but was coincidentally just thinking about this and saw this topic.
If I had to put it into words, modularity and non-linearity. I've always liked deep customization in games, the idea of socketing unique stat bonuses on items and whatnot.
Awhile back I found these two games, Railroads and Catacombs and Wantless. They both have these really open ended customization not on gear, but on the abilities you use. Well, Wantless also has it on gear/equipment a bit, too.
I'll try to use a somewhat common example. You know in Diablo 3, how you can select different affects for your abilities that you use throughout the game? One may enhance the duration of an effect, one may increase its damage but reduce its range, one may reduce the range but come with some other tradeoff.
Well, Wantless allows you to make abilities from scratch. You select the area it affects, be it if it's yourself, or targeting an enemy directly in front of you, or at a distance. You determine the width, if it's one tile or multiple.
You select the effect. It can be a heal, a buff, a debuff, whatever you can think of.
And then you can mess with the damage and duration or whatever else you want. It's only limited by your own progression tied to a resource pool, so you can't completely break the game. But you may choose to have one or a couple insanely heavy hitting nuke abilities with a long cool down, or you may have several that perfectly accommodate each other's cool down, which you can also choose. Higher cooldown=increased affect, which I also thought was a really brilliant design choice.
Railroads and Catacombs is similar but you don't choose as much, you are randomly offered a choice of abilities to choose from, and then add your own modifiers from a preselected pool. I made a really unique, in my opinion, ability that applies a shield to myself every time I move, as well as does damage to everyone around me. Even if it deals damage to myself, the shield absorbs it.
I just think modular skill making like that is absolutely the future of RPGs. WoW dabbled with that with glyphs and then that necklace in one of those expansions I didn't play. But to completely choose your spells and abilities, from the ground up, is just insanely rewarding and challenges the part of me that loves min-maxing and theorycrafting in games to see what is possible.
Also, non linearity. This is seen in series like Fallout a lot for example. Gothic is my go to example though. When I played that game for the first time, I killed some wolves that dropped some item that was tied to a quest I never even picked up. Fifteen minutes later I met the quest giver who's like "Those damn wolves, I wish someone would take care of them so they aren't a threat to anyone on those roads anymore." or whatever. Instead of just accepting the quest, THEN the item spawns on them when they respawn.. And instead of accepting the quest like normal, exiting dialog, then clicking it again to immediately turn in, no.. The immediate response in dialog is like "Oh, those wolves? I already took care of them. Here's the proof." quest complete, right then and there.
You can go off and explore wherever you want to without any real consequence or missing out on anything. You'll often pick stuff up that you don't know what exactly it's used for, yet, but you're likely to find out later, and you get a nice reward for it. It boggles the mind how all games don't do this. It's intentionally made more tedious by making you have to KNOW where the quest giver is and go to them and start it, versus just retroactively having all the quests be "active" in a way, but hidden.
Just really, truly brilliant design in my personal opinion, and I desperately hope it becomes the norm.