New Campaign and World: Planet X

It's been a few years since I've run a campaign in earnest, and I'm looking to get a new one going. I have 2-3 potential groups right off the bat that I could run games for, so I want to get a decent amount of material together to get started.

System: The base system I will use for this is the excellent fan-made Gamma Five rules by Emanuele Galletto. I really like the streamlined character generation, and the fact that its basically 5E but even simpler.

Setting: The overarching idea for the world is that at some point in history (probably today plus or minus 20 years), the earth was devoured by another planet. Most of humanity, and the entirety of the world as we know it was wiped out. The result was an entirely new planet, henceforth referred to as Planet X. Fast-forward unknown aeons, and this new planet is inhabited by a mix of mutated life forms from the original two planets. The surface is largely primitive and wild, and there are underworlds made up of the lost ruined cities of past ages.

Characters: I will do away with plant-men and robots, but use all the other character bios (avian, beast, insectoid, mutated human, pure-strain human, and saurian). The idea I have for pure-strain humans is that they are hatched from ancient eggs. The ancient humans knew of the impending collision and stored as many embryos as they could in these biotechnological wonder-eggs that could sustain their lives and allow them to hatch when conditions were safe enough.

Influences: I've been reading some Clark Ashton Smith lately (the two Xiccarph stories, as well as Vulthoom), and I've also been watching 80's cartoons to get that beautiful science-fantasy vibe. These include He-Man, Blackstar, Thundarr and Silverhawks. Here is some additional inspirational art below...