Path of Exile.

Its 100% free, so you can play it like a demo until you are either fully in or bored.

It gets a season refresh every 3ish months with an entirely new mechanic, so most experienced players restart it 4 times a year, play a new build, and then stop playing when bored and return at the new season so its always got reasons to peek back in and see if you want to rejoin.

The leveling system is a little daunting at first, but the base game is completely playable newbie blind even if you pick "suboptimally" from the skill tree. If youre too worried about FoMo, there are tons of guides online for builds, both top level and newbie friendly.

The attack/skill system is very interesting and completely moddable, tons of replayablility.

If you like diablo, try PoE.