Still haven't had your favourite codex updated? Or seen your favourite 40K faction get a codex ever, even after all these years - if not decades? Never been offered a simple method for making a whole new one? After all, the galaxy is a very big place.
Here's an idea to experiment with while you wait.
First, think about the faction's nature, then their motivations, means and methods. Write a bit of background, draw a unit or two, convert some up.
Then for each entry in the list do the following...
- Pick a) one statline then b) one set of equipment and/or c) one set of special rules from any three existing entries in any codex. If you pick a), b) and c), find the points value for one model for each entry, add the three values and divide by two. If you pick a) plus b) or c), add the two values and divide by 1.6.* Round up. Then pick d) one unit size or number per slot and e) one set of available upgrades, from any of these same entries.**
- Decide which slot the unit will fill. Each slot should have three to six options.
- Name it. For a quick familiar vibe, you could choose the words from this list:
angel; apostle; auto-; auxiliary; aveng|er/ing; axe; bad; bane; banshee; barge; battle; beast; beserk[er]; betray[al/er]; big; bike[r]; bile; black; blade; blast[er]; blood; bomber; bone; born; boss; bringer; brood; brother; brute; buggy; cabal; cannon; chain; charge[r]; chem-; clan; claw; command; coven; crusade[r]; cult; cy[-/b|er] d[a]emon; dancer; dark; death; demoli|sher/tion; destroyer; devil; dire; dog; doom; dragon; drake; dread; drop; eagle; emperor; engine; eternal; evil; falcon; fang; far; fate; fear; fiend; fighter; fire; fish; fist; flame; flesh; force; forge; fury; fusion; ghost; glaive; grav-; grey; griffon; grot; guard[ian]; hammer; hand; hawk; head; heart; hell; helm; hive; honour; horror; howl[ing]; hunt[er]; imperial; ion; iron; jester; jet; knight; lance; land; legion; lightning; lion; lone; lord; mage; marine; master; maul[er]; mega; mind; mirror; moon; nemesis; night; nova; pain; paladin; panther; penitent; phantom; phase; phoenix; plague; plasma; pod; power; priest; prism; psy-; pulse; pure; quest; raid[er]; raptor; ravage[r]; raven; razor; reap[er]; reave[r]; red; rider; rip[per]; rune; runt; sabre; scar; scorpion; scourge; scythe; seer; serpent; shadow; shard; shark; ship; shock; silver; sister; skull; smith; son; spectre; speed[er]; soul; space; spear; spider; spirit; splinter; spore; star; steel; stone; storm; strik|e/ing; suit; sun; swarm; swoop[ing]; sword; talon; tank; tech; templar; thirst[er]; throne; thunder; tide; tiger; tomb; torment[or]; traitor; trooper; tyrant; vampire; vector; vengeance; venom; void; wagon; wail[ing]; war[rior]; warp; watch; wind; wing; witch; wolf; worm; wraith; wyrd
Finally, pick any two army wide-special rules, finessing a bit if necessary, a traits table and disciplines - and you're done. That should be enough to start playtesting. Of course, if all of your players are using this system, you might not even need to worry about that.
* This is a lot easier than it looks, even mentally: 1.6 is just 5/3 (because 0.3 is 1/3) and dividing by 5/3 is the same as multiplying by 3/5, i.e. multiplying by three then dividing by five. Common fractions are very useful.
** If you'd like a little more restriction here, you could find the mode of the entries, or the median otherwise._