The Battle for Wesnoth is one of those oldies but goldies when it comes to Linux gaming.
I'd also heartily recommend trying out OpenMW (not just because I'm part of the team there) as a modern way to play Morrowind, though the upcoming release is not entirely finished, there are a few blockers being hammered away at still.
Seconding OpenMW! Don't let the upcoming release being unfinished dissuade you from playing! I played through the whole game back in 2015 with OpenMW on a crappy laptop with about as many bugs as is normal for a Bethesda game.
The did great work to get it to that point back then, and I'm sure it's even better now.