HL2:Episode 1+2 are continuations of the story after the events of HL2. It took me forever to realize that HL1:E1 wasn't the first half of HL2 lmao, they're actually sequels. They should have called them "HL3: Ep.1/Ep.2" to be less confusing imo.

Deathmatch: Classic is Quake's multiplayer in GoldSrc, and Deathmatch: Source is HL1's multiplayer but in the Source engine. They're not worth bothering with tbh. The Quake remaster and the original HL1's multiplayers are way better than those games, I highly recommend them if you're into chaotic arena shooters. (Only if you're willing to get your ass kicked while learning, though ;P)

Tbh Opposing Force and Blue Shift aren't the greatest, but they feel like very high effort mods which is about as much as I could as for. I recommend trying them but I wouldn't feel too pressured to stick it out all the way through if you don't feel like it. Their stories are trivial, and the OF protagonist never even comes back. But HL2:E1+E2 are pretty good, albeit flawed compared to HL2. Highly recommend them in their entirety. HL:Alyx is also great for what it is, a AAA VR experience. I highly recommend that one too.

Black Mesa goes beyond being a simple HD version of HL1, the xen world is brilliant in Black Mesa

Gordon invents time travel by crowbarring a combine thing.

Sorted that plot hole out pronto.

Thats...gonna be hard.

Nonsense! There's so much room for possibilities. The Borealis was an Aperture invention, and HL2:E2 ends with the search for it. The setup is there for a Portal / HL crossover episode.

And Chell doesn't have to make an appearance. We know of at least 1 other surviving subject from Portal 1 and Portal 2 opens with Wheatley relocating subjects in the same form of stasis Chell was put in. Plus >!the subjects that the robots discover!< at the end of P2. Maybe there's >!more vaults, and maybe they could make an army!<? And you got the cores that can talk and some that seem to have AGI...

But honestly you don't even need any intelligent beings. Aperture itself stayed safe ostensibly for >9,999,999 years after the war, and many years before it. Parts of Aperture could serve as a hideout, especially if they find the Borealis to get in and out of there without a trace. Laidlaw said in his Epistle >!that the intention was to use it for combat!<, but I don't see why they couldn't spelunk Aperture for some weapons and tools first.

All it takes is one pair of moon dust panels to drain an entire planet's atmosphere of whatever an alien needs to survive on their planet, and more could do it faster... Or maybe they could tinker with the portal guns to shoot bigger portals while Gordon goes on a side quest or something. Then tele in on the Borealis, set up an array of panels and portals, tele out and open the corresponding portals from a distance in space. There's your climax of the episode, important Combine planet #1 is in peril and while Earth is probably in danger of a counter-strike (hehe), they're in an even better position to respond and defend than they were before.