There are literally thousands of other RPG systems.

If you want to stay close to the D&D experience, the big alternative is Pathfinder of course. Pathfinder 1 was basically an expanded/fixed version of D&D 3.5, but Pathfinder 2 is really its own thing and possibly a better direction than D&D5 is. But more importantly, Pathfinder is legendary for its massive thematic campaigns. (There are some duds, and none are perfect, but many are quite good and there are a lot of them to choose from.)

Another option is 13th Age, by Jonathan Tweet, one of the main developers of D&D 3.x, and according to some people, it's the system that D&D should always have strived to be. (I don't know it myself, though.)

Then there are tons of retro-clones trying to bring back the experience of older D&D editions, including:

* Dungeon Crawl Classics

* OSRIC

* Labirynth Lord

* Swords & Wizardry

* Old School Essentials

* Whitehack

And many, many others.

Some modern narrative-style systems:

Dungeon World uses the Apocalypse World system to simulate the D&D experience.

Apocalypse World itself is a post-apocalyptic Mad Max style game, with a very different, light-weight, narrative-focused system that's great at simulating different genres. There are spin-offs for almost every property imaginable, including Cthulhu, Star Wars, Cyberpunk and many others. The game itself is not generic, but specifically tailored for a specific feel, but the same approach can be used to create games tailored to different genres.

Blades in the Dark is a bit like that too, in that it's been modded for all sorts of different genres. It is however specifically tailored for heists. The original is a kind of pseudo-Victorian fantasy.

More traditional RPGs:

Shadowrun is cyberpunk + fantasy. Near future, dragons rule the boardrooms, corporate mages defend their employer's interests. And for plausibly deniable direct action, they hire teams of mercenary criminals (the players) who basically do heists. Violent, rich background, complex system, poorly edited rulebooks.

Cyberpunk 2020 and Cyberpunk Red: the system the Cyberpunk 2077 CRPG is based on.

GURPS is a system that's able to handle any genre (or so it claims, at least). Very generic, and if you want to mix cyborgs, wizards and superheroes, in space, this system can do it.

Savage Worlds: also generic, but lighter than GURPS and optimised for mass combat. You can have massive battles in this system while still remaining playable. But it's mostly used for pulpy action.

Star Wars. Of course there are several Star Wars RPGs, but the best ones are the original WEG d6 Star Wars (which created much of the expanded universe lore of Star Wars; Lucas Arts loremaster Pablo Hidalgo got his start there) and FFG Star Wars (Edge of the Empire, Age of Rebellion and Force and Destiny, uses weird custom dice, but great system).

Traveller -- the original SciFi RPG. Must be up to about a dozen different editions by now. 11,000 worlds, millennia of history, aliens, spaceships, but often ends up focused on trade. In the original edition (and several others) you could die during character generation.

There's hundreds more. I'm not going to list them all; this is just a random sample.