What do you want, specifically?
More immersive sim elements. Environmental interaction and reactivity. Opportunities to do things with the skills and spells I taught my character. Fun skills and spells to use on the world.
The more simulationist a game is, the better. Fewer abstractions, more simulations. Fewer simplifications, more complexities.
A proper RPG should let me do things. Quest design should be open-ended rather than prescriptive: design quests around a goal that can be reached in any way you want, rather than forcing you to do it in steps.
Good: "Your water filtration system has failed and you need a replacement chip. Go find one."
Bad: "Your water filtration system has failed and you need a replacement chip. First, go to Vault 15 and ask them for one." and only after you visited Vault 15 and didn't find one, you can go to the next place and ask there. This fucking sucks. Fallout is great because you can skip a lot of steps once you know what to do, all that matters is reaching the goal.
In a similar vein, quests you can solve before you get them. And quests that you can fail before you get them. Tamriel Rebuilt has some of these, where you can get a quest to capture the soul of a unique monster in a soul gem, but if you already killed it without capturing, the questgiver will tell you that you suck for doing so. Having your actions in the world carry consequences even when you don't know or expect it is super cool and shows the devs paid attention to details.
Which things did you like about old games that never had a chance to evolve, progress, or come back?
More detailed and layered equipment systems, like Morrowind's. There are a few games that do this, like Kingdom Come Deliverance, but not to the same extent as Morrowind, and most games have the same copypasted equipment layout of helmet-breastplate-gloves-boots. Let me wear a shirt underneath my armor, and a robe above it, and two different pauldrons for cool mix-and-match armors! This is not only cool for dressing up your character in a unique style, but also makes loot more rewarding, particularly in a solo action RPG. With a single character, adding more equipment slots means more of the items you find will be useful. If you only have 4 armor slots, they'll be filled quickly, and anything you find from now on has to be a straight upgrade to be worth picking up. With over a dozen equipment slots, you can go much longer before filling out your optimal armor set.
The ability to use any skill on any object in the environment, as seen in Fallout. With the skilldex, you could simply choose a skill and use it on whatever is in front of you. Click on the repair skill, click on a broken well, your character attempts to fix the well. Nowadays, interacting with objects usually involves clicking on the object and getting a dialog window with several options to choose from. Age of Decadence is a prime example of that, all interactions happen through dialog windows. Fallout's skilldex is a lot more elegant and requires you to think up the solution yourself. The game doesn't just tell you "Hey man, you have the option to repair that well", it only shows you a damaged well, and if you put some skill points into repair you might think about trying to repair it. Fallout didn't use this feature to its full potential, and no other game since has picked it up. The potential this has is insane.