Pitch the game you don't have time to run

iirc it's mostly the same as Mongoose 2e with the notable differences that there's obviously no Empire, galactic space is divided into real world first power countries. There is no antigrav technology, and so there's an added element of low gravity sickness, and gravity experienced during ship acceleration, TL is limited to max 12 (I think?), and psionics. Those are the main things I think I recall from the top of my head. But for the most part mechanically the same system, just different lore and added restrictions.

I've actually had a look at all of the Zozer games Hostile supplements, and some of the Cepheus engine stuff and tried very hard to put it all together in the past... and just couldn't really get it to work out in my head the way I wanted it to. The Pirates of Drinax supplement had some base building stuff that I think could work out if I really forced it to, but finding a way to scale it up that isn't just a linear translation is the challenge. I also have a tendency to flounder when my projects are too sandboxy, either from my own issues of worldbuilding bloat and eventual burnout, or not pushing players hard enough to get past their own choice paralysis.

I was also on a mecha kick when I dreamed this all up and the custom rules for creating a vehicle, especially with walkers, makes my head spin.

I think I at least had a pretty good explanation for the walker vehicles in my setting, as they're typically worse in general at combat compared to traditional military crewed vehicles like tanks (especially grav tanks) and gunships.

My reasoning was in regards to tonnage displacement and the configurability actually make the walkers ideal for shipping to frontier areas as industrial equipment. And would be the primary set of vehicles the colonists would use to defend themselves against threats on the frontier, and require less crew (only 1 seat to control).

Might I humbly suggest Savage Worlds? It's easy to learn and run, and with the new Sci-Fi Companion, making ships and mechs is easier than ever! It works beautifully for science-fiction campaigns. I'm building a space opera campaign for my players right now.

A Warhammer game where players aren't Imperial subjects in the Imperium. Just for the change of pace.

I've run a 2e game using Tome of Corruption, where the 4 Kurgan players were half brothers (to give them a reason not to immediately kill each other) and each was a champion of a Chaos God. We began with them as lieutenants of Asavar Kul, fleeing the siege of Praag and chased by Kislevite Winged Lancers. Their first order of business was to survive, repel their chasers and regroup their troops. I then had them collect 5 artifacts (1 for each god plus 1 of chaos undivided to hold them all together) to make a banner that would rally troops to them and the rest of the campaign was them building up a host that would eventually descend upon the Empire once more. It was quite refreshing and after discussing it with my players, there were two main reasons for it. One, instead of dreading mutations, they welcomed them as a blessing and it felt new. Two, they were woefully inexperienced and underequipped but still had to go about being all macho to save face and inspire troops, so they kept pushing the envelope to suicidal lengths, at time failing spectacularly (they'd all been "resurrected" which basically meant rolling a new character that also retained a few features and the fractured sanity of all their previous ones) and other times hitting the ball out the park thanks to lucky rolls.

Seeing how that worked, I'd like to try a few other ideas out of the ordinary Warhammer experience:
1) A sandbox/hexcrawl type of campaign set in the Border Princes, where the players do a bit of domain management/expansion too. Think Birthright or Pathfinder Kingmaker.
2) A "crusade" type campaign where the players follow the Order of Knights Panther into Araby.
3) A "trek into the New World" type of campaign where players first land upon the shores of Lustria.

My mind immediately went to Black Crusade just before the next sentence you wrote where it became apparent that you were talking about Warhammer Fantasy and not 40k.

Oh yeah, my bad, should have clarified that from the start. I'm much more into Fantasy than 40K, although I've played Dark Heresy, Deathwatch and Black Crusade already. Rogue Trader still escapes me, but maybe the videogame will scratch that itch...

Chinese wuxia monster hunters, with conflicting feelings and obligations for personal drama.

Everyone has at least one active element (wood, fire, earth, metal, or water) to enhance maneuvers into supernatural levels.

@offbeat.lemur

If you haven't settled on a system, I'd highly recommend Legend of the Five Rings, and it plays quite well into exactly this. Or if you want something less crunchy, you could try Hearts of Wulin, which is PbtA-based, but has a really fun Entanglements mechanic for social discord.

A name like that reads like a PbtA retry of Legends of Wulin, heh.

Well, definitely not L5R, that comes with so much overhead I would use Fate for a Rokugan game.

I'll check out HoW, though, it seems spot on for this idea.

Not really a campaign but a series of one shots. I tried to do it just before I had to take a long break from GP, but I would like to try running some one shots that are a minimum level of 15 in d&d 5e to see how epic things could get. Not easy, especially when the rules are not the easiest to follow that high levels and suddenly playing with people you don't know well who may have expectations wildly different than yours makes the beginning rough