Well, now for version 1.1!
http://www.scribd.com/doc/32337428/Feral-and-Medieval-Worlds-for-Warhammer-40k-Rough-Draft Taking MekanobSamael's suggestion to be more specific, I want to make the Feral Worlds sort of like the oldschool TV show "Land of the Lost". No there's not going to be Sleestak but I really want to add in different colored crystals and whatnot.
There will still be adventurers, they just haven't been added yet. Note that I have added in Aliens. Why? Because I felt like it. It adds some very cool stories to the game. For example, the Imperial Army is patrolling a Feral World, attempting to bring it back to the Imperium, and BAM! The encounter several squads of an unknown alien race. Hell you could make a campaign out of it.
Personally I have having a really fun time with this codex. Most fan-made codices that aren't based off the big factions always include these super high-tech, advanced modern societies. Yeah, well your Imperial Marines are fighting against Savages with Clubs now instead of "hyper futuristic dudes who are more advanced than Eldar".
Yay tangents!
Anyway this is still a rough draft and it still looks a lot like one. But I'm polishing it up. Any suggestions?
:edit:
The Aliens are very fun to play as. One game for me they were WS2 BS5 S2 T3 W1 I5 A1 LD10. I had given them Bolters and Carapace Armor. They were kind of like Tau, haha.
By the way, people who are worried about randomness: It's before deployment, and in a way it's like "create-an-alien" in that you get to choose where the dice end up. Sure you're kinda boned if you roll two 1's for your d3's but they're only 5 points each so it's not that bad. Plus even if you do get 2 1's, I'm sure that you'll get some better numbers for your other stats. The chances of you getting all 2's and below is so low that I'm not worried about it. The same goes for the chances of having Aliens that are WS4+ BS4+ S4 T4 W1 I4+ A1 LD8+. It all averages out in the end. Let me know if you can think of something better, though.