I'd say Power Up, On a Rail, and maybe Questionable Ethics would be nice canidates as they all are probably on the bottom-tier of recreational difficulty when compared to, let's say, Surface Tension (but the validity for On a Rail is something I'm not too sure on).

As for suggestions as how they could be made, Power Up could be a level similar to Nova Prospekt or a run-down rebel base for either of the two canidates for the Garguanta: Antlion Guard and Strider respectively. Antlion Guard should have its health higher, but still killable to commemorate the Garagantua in Power Up to be killed. The Strider doesn't need much applied to it.

On a Rail could be based off of Water Hazard as the controls for the Airboat is the most limited vehicle in the entire game (besides the Pod). Otherwise, it could also use the Razor Train as a replacement, but I'm not too sure if it's easy to code.

Finally, Questionable Ethics could be based off of White Forest base, where everything is now taken under Combine control as a recall to the situation of Advanced Biological Research Lab where the HECU has taken over the lab. Also, to commemorate the Tau Cannon, how about the Gravity Gun turns into the Super Gravity Gun, and lasts until the player has fired the giant laser into a wall.

I would include more but then this post would turn into a giant wall of text so I only put in what I thought would be the most interesting to recreate while simultaneously on the easy side to recreate.

For any three of them, I'd recommend using the Black Mesa: Definitive Edition remake as it HEAVILY improves it

surface tension could be cool

Black Mesa Inbound /j

wait maybe xen chapters could be like the combine overworld

Office complex

Originally posted by Dapple Dualies:

wait maybe xen chapters could be like the combine overworld

Sadly xen in half-life 1 was made to be played with the long jump modul, and it's not present in half-life 2

office complex

unforseen consequences

Originally posted by Centurion:

unforseen consequences

agreed

surface tension brahhhh

office complex would be fun and it would look cool

hmm... if you do make surface tension i think it could take place during the uprising? idk i think thats a good idea, dunno how hard it would be to make but, its a possiblility!

or you could make blast pit as a sequel idk.

I think i'll start with office complex first