What about DIM - LOC - 666, it is a very hellish landscape with a lot of fire, the combine's purpose there would be to borg-ify the extremely hardy, extremely rare creatures. the structures would be elevated, as to not have anything catch fire, and they would use large, heavy duty traps to capture wildlife

Or DIM - LOC - 3110, a very dark planet somewhat like Umbra from Star Wars: TCW. the combine would be present because of an abundance of valuable natural resources. there would be a standard combine fortress with a citadel.

I'm a fan of your second idea very much.

DIM-LOC-X The Combine occupied Dimension X
Combine are there for research and mining

DIM-LOC-4793, the Vortigaunt homeworld.

It has very lush and colorful plantlife with the combine architecture as a steep contrast. There's pods which the Vortigaunts step into to siphon out their energy which litter the once great Vortigaunt city. Now this is where the headcrabs are from so you could have the cut combine big momma pod in it.


DIM-LOC-294, the Race X homeworld.

This planet is similair to the antlion homeworld map you made except Race X is intelligent and can build stuff. There's many spires and the like. Not much to say about this one.

The Vortigaunt homeworld is a guessing game, and also it would not be just for the Vortigaunts as they are not the dominant species of the similar world they co-exist with the Nihilanth and such.The Vortigaunts are not native to Xen and do not actively live on Xen like the other fauna that refuges within Xen, it's clear that the animalistic types of the wild-life are for farming and combat reasons, As alot of the ''Xen creatures'' are from the same planet, which you had nailed on the point of the Headcrabs co-existing within that planet. However one grudge on the request from me is the explanation of the ''Great Vortigaunt City'', due to the fact they are the lowest members of society, being slaves and the like to the Nihilanth Creatures, and the fact that the highest members of society are beings which can float and have a very large mass to them, however I do think Vortigaunt labour areas or areas which Vortigaunts thrive within would be a thing, similar to that of the Xen factory being an example.

The Race X homeworld is some risky business, as we aren't sure how they live in general, we can guess though. I doubt the Race X aliens would live on a very deserty overworld, due to the fact that Baby Shock Troopers are incubated within containers of water, giving the notion that they may be amphibious, also their appearances certainly don't match a desert environment in my opinon, however, A Race X homeworld is a purely guessing game like the Vortigaunt homeworld, and all we can do to shape their homeworld is by analyising their appearances and behaviour to shape a map.

DIM-LOC-4592
A marshland that was once an ocean, it has one huge "island" (now a mountain) with a citadel that errupts from it's sprawling depths, it is believed that this is the world that they would recruit their "flyers" such as dropships and gunships. The ocean was filled with a calcified "slurry" of corals which the combine have mostly dried up (thanks to the evaporation of most of the ocean and the extermination of most of the aquatic life to make way for vast synth factories and mining facilities).
This calcium and iron from this world is usually molten down to feed synths, and many of the synthetic creatures raised here are shipped to various other dimensions like the combine homeworld and earth for the construction of better synthetic units for use in risky areas that are prone to resistance from native beings.

DIM-LOC-43110

DIM-LOC-43110 It's a very irregular planet with an infinite and super-dangerous storm and has a lot of caves in the surface,also there's no gravity on this planet so there are a lot of floating islands. When combine noticed about this planet, they decided to build a great energy plant so they could use the storm as a infinite power source, their main base in that planet it's a floating island with a giant glass dome with a small citadel in the middle, equipped with five lighting rods at the top and a elevator that goes to the surface.The energy taken from the storm, it's later transported to citadel engines and there, used in multiple machines to make them work, the glass dome it's also surrounded by giant combine spaceships, for researching of more resources at the planet, this floating island also has a portal in the core, the fastest known combine portal. Planet surface is a flat grass with many hills and a lot of cave systems.This planet doesn't have any advisors,it's runned by one combine soldier, which is the one who discovered this planet and decided to place there a citadel, this soldier is known as the black overwatch elite.

DIM-LOC-43110 utilities:

-Used as energy plant, combine can power a lot of machines
-Because of storm energy, this planet has the fastest combine portal
-Combine also transports here the most powerful resistance members and if they don't say anything during the interrogation and advisors think they're useless, combine drop the rebels outside the dome

Landscape:

-Flat grass with lots of caves and hills
-Infinite and dangerous storm in the sky, which is blue.
-There's only water inside the caves, also, its blue and has a pretty good taste, this is the most rare resource to get.
-Green sky, but it can't be seen cause of the storm
-Floating islands everywere
-Some trees

Life inside the planet:

-There aren't any known animal species inside this planet, only some kind of bacteria inside water and flying ones in the storm
-Planet has a great flora in the surface, but it's not recommended to get close to them, they are strong and can beat a strider if he gets close, but the ones inside the caves are pacific and small. Trees are the only plants that aren't alive, but all the plants have in common that they are resistant to the storm

Currently status:
-Working

I'm loving the ideas guys, posting them in my lists and keeping track of them and thinking them over! Keep it up!

DIM-LOC-1897
The landscape is for you to decide, although a gassy, green, and aerial(1) environment would be good.

The birthplace of gunships(2), or at least the creatures that are turned into gunship synths. The combine would have invaded without much effort, since an aerial soldier, mortar synth, advisor, or any other creature would be capable of eliminating them. After all, gunships are pretty floaty even with the synth attachments, so normal ones probably aren't very proficient fliers.

According to the wikia(3), "They are officially described as 'self-replicating robots that evolve.(4)'"
This would mean that after enough are taken and perfected, they would have no reason to continue the invasion, other than resources, which would eventually be depleted.

After all of this, the planet would be left a husk, littered with abandoned Combine machiney, processing, outposts/checkpoints, and anything else.

this idea was too good not to post even though this is like 2 months ago

footnote (thats what those numbers were)
1: in a similar manner to Xen, but much flatter and with like 40% less cave
2: gunship eyes have a small resemblance to crab synths and mortar synths, so it could also be the homeplace of them.
however, the strider and gunship have the same logo, so they're more likely to be from the same planet
3: http://half-life.wikia.com
4: in the citadel you can see a gunship factory, further proving that they don't need to be taken from their home every single time

It's never too late to write down an idea, always open to ideas and putting them into my list - This idea is pretty cool to explore, as it is centered around the Synths we see often - I'll definitely put this idea into consideration, however i'm in abit of a dilema with my maps which i'll put up on my group due to my lack of encouragement to continue on with a certain map series.

Maybe Antlion ?

Antlions are from Xen, thats why they fly temporarily on earth.

What about a planet that is entirely organic. By this I mean that everything would look plant-like or fleshy, and would occasionally move around. there would be many combine structers that would would be partially sunken, or overtaken by the planet.

Well, Xen is much like what you have described, it is pratically made out of biological matter, it's even hinted at the giant bacteria in the HL1 Skyboxes and even just generally how everything looks, however I believe you would need to explain more of why the Combine would be on this Organic planet.

Snjar maybe it was a race that was in some ways superior to combine in combat or stuff, so they had to take over it and kill everything to get rid of it.