So, for many things there aren't alternatives (or active ones, at any rate). The Threadiverse userbase is presently considerably smaller than Reddit's. Reddit has about 430 million MAU. The Threadiverse has about 50k. For every active user here, there are about nine thousand on Reddit---that limits our coverage of niche stuff.

That being said, I'd recommend looking at:

https://lemmyverse.net/communities

They spider all Threadiverse instances and build a searchable list of the communities out there.

Note that by default, they don't show NSFW communities (you'll need to tick the checkbox for it), which might be an issue if you're looking for analogs of subreddits that have pictures of girls or similar.

'Course, each additional user expands the ability to cover more niches, so welcome. :-)

EDIT: You should be able to find, for example, !leagueoflegends@lemmy.world on there, but also notice that there's very little posting outside a moderator posting the patch notes for each release.

EDIT2: You'll get more traffic on more-general communities like !games@lemmy.world, if you're hunting for activity, and they generally aren't overwhelmed by traffic. But...they also won't have the same level of focus as on communities specifically focused on individual more-niche topics.