I think !battletech@lemmy.world is pretty neat. A setting with webs of overlapping political intrigue that shifts between different eras, all driven by conflicts where big stompy mechs are used to fight. It’s got decades of different miniatures behind the actual game, and the game itself is very rules crunchy. The small number of mechs needed to play makes it quite accessible compared to other wargames that need more of an investment.

!warhammer40k@lemmy.world is another setting, probably more well known in the current mainstream. A huge variety of factions to choose from. It is the default game of any game store so you’re sure to be able to find other people to play with.

!star_wars@lemmy.world is a setting that’s had a lot of controversy, but I personally like focusing on all of the great parts of it. I’m currently reading through some old EU books and really enjoying them.