Have You Played... MINERVA: Metastasis?
Have You Played? is an endless stream of game recommendations. One a day, every day of the year, perhaps for all time.
As we all know, Half-Life 3 will be released without fanfare tomorrow, but if you just can't wait any longer for more dabbling with the Freeman arsenal, there's always MINERVA. Originally a mod but subsequently granted Valve-endorsed standalone status, this was the Half-Life 2 campaign that did a number of things better than Half-Life 2 itself did. So much so that Valve wound up hiring the guy who made it.
By far the greatest trick up MINERVA's sleeve is smart and efficient level design. Where Half-Life 2 was divided into a steady stream of loading points, MINERVA is far more economical about its use of space. You don't simply go over and along, but under, through, downwards, twisting, turning, circling, kept within a world dense with encounters and occasional puzzles, without it ever feeling cluttered. This is how watchmakers would design FPS maps.
Meanwhile, an unseen narrator of opaque intent leads/orders you through this Combine-patrolled maze, doling out snatches of secrets and hinting at darker purpose. Though more sinister and stranger than Gordon Freeman's melodramatic adventures, MINERVA is probably as close as it gets to a bona fide extra slice of Half-Life 2.