Half-Life 2

After having bought (and played) Black Mesa, I thought I might as well get Half-Life 2 (or HλLF-LIFE² as it's stylized) and experience first hand how Gordon Freeman's journey ends. The game's second standalone DLC, Episode Two, which was released in 2007, teases another sequel but Valve has not been willing to create one so far. In 2020, they did release a virtual reality game called Half-Life: Alyx (stylized with an upside down lambda as y which I'm unable to reproduce in text) but its events are concurrent at best (though the game does add to Episode Two's ending a bit).

Portal 2

I felt Portal 2's single player campaign was mostly a reboot of the original game, even though it technically is a sequel. But somehow you (Chell) are back in Aperture's testing chambers and it's time for another breakout, since you incidentally also cause GLaDOS to revive.

Portal

This was a triumph and the cake is a lie. And yes, I did indeed play Portal three and a half years after it was released.