Joshmohr:
Im the Tyranids, skinned as the Aliens form Aliens!
Auretious Taak:
Pick something tiny from an armies background and go from there.
How about you take an Assassin Army.
Hear me out, you take an Assassin conclave/house, and you’re obviously taking say a callidus assassin (don’t do the vindicare, boy does he suck as a whole, unless you take a crap tonne of snipers), some death cult assassins (assassins in training) then all the support staff an assassin academy would need. Inquisitorial codicies would be the obvious start here, witch hunters I think would work. Take Arco-Flaggellants as training targets, Inquisitor Lords as high exemplars or some crap from the academy or something like that and then a crap tonne of storm troopers to represent menials/the stand ins for all the training requiring targets to practice assassination on. Heh.
Could work, could work well.
dedwrekka:
“Guardsmen”
-Hive Militia (Gangers, Arbites, Fanatics and some PDF)
-Crusade Army (Multiple units from different armies, gives a chance to experiment and make unusual combinations)
-Grot Rebellion (Kreed and Keel say do it!)
Eldar
-Harlequin army (Fully converted for every member to be a harlequin)
Necrons
-Army of the Outsider (The C’tan who started to eat the other C’Tan, and went crazy for it)
Orks
-A Full Waaagh!
-Madboyz (Orks gone crazy on the edges of sanity)
Tau
-Using rules for Tau, make your own race, explaining them as an alien race within or allied with the Tau Empire.
I have another idea, but if I say it, people will steal it, and you’d see it everywhere!
snowblizz:
dedwrekka, I demand a larger pic of those Grots!
slev:
Thinking back over all the old “never made” and “fallen-by-the-way-side” armies (some metioned already):
Squats
Genestealer Cults
Exodites
Hrud
Demurge
Slaan
Feral Orks
Feral Humans
Chaos Renegade Orks
Eldar Pirates
Ork Freebooterz
Tech-Guard
Adeptes Arbites
Adeptes Custodes (Heresy-era)
Sisters of Silence (Heresy-era)
Chaos Cultists
Harlequins (full army)
Ratlings (full army)
Ogryns (full army)
Kroot (full Army)
Vespids (full army)
Cursed-Founding army (other than LotD)
The Fallen
Thommy H:
I always wanted to do an Arbites army. Came within a watched ebay item of doing it once too. The Witchhunter Codex gives all sorts of nice options for doing them.
Tarrakk Blackhand:
@Slev - are there any pictures of these armies/figures out there on the web?
Thommy H:
Google is your friend, Tarrakk.
Tarrakk Blackhand:
I’m a Metacrawler man myself! 
Yeah, I guess I could open a search engine or 2…but I’m a bit lazy.
slev:
Tarrakk: I have no clue, I’d just google them myself.
If you need general ideas on them though, just ask.
Boomer:
Hi
I’m new on here but i’ve been modeling and converting themed 4ok armies for a while.
So far the two i’m most proud of are Humanid army which counts as Tyranids and uses human (mostly I.G) bodies with Tyranid bio weapons sounds a bit funny i know, here is a example of them
.
And one of my Zoats.
My second themed army is my Khrave and Barghesi army.
I use the Ork codex and have converted the entire army to look like evil chaos loving xenos. The pink guys are the Khrave and the blue guys are the Barghesi. 
Anyway i hope this inspires people to have a go at something different.
Boomer.
Kera foehunter:
nice army boomer
This message was automatically appended because it was too short.
Tutankhankh:
How about Gue’Vesa? That is, a human army, loyal to the Tau, using Tau equipment and markings.
Nicodemus:
How about going with daemons bound within weapons, armour, machines… but not just any run-of-the-mill daemons, but ones that were too powerful for those who tried use them, and have taken over control of their former masters. Maybe things like cannons, bolt pistols, swords, power fists, etc. that have partly grown over the person wielding it. You could easily mod the weapons to have horns, faces stretching out of them, or other daemon bits like arms or mouths, etc. Painting them I’m guessing you’d want to go with bright colours and lots of highlights to emphasize the daemonic parts, but the units carrying/wearing them you could paint with duller colours to de-emphasize their presence… and maybe even zombify them a little.
You could easily take any mix of units from any army and just daemonize their weapons or other equipment and mix them all together, perhaps representing that these daemons have organized themselves, irrespective of the different races that was trying to use them. Or maybe this is not a collection of different daemons, but one ultra-powerful daemon who couldn’t manifest himself in the material universe but lent some of his essence to some corrupt chaos worshipers to create daemonic weapons, but he then betrayed them and seized control of them through his new-found physical presence. In this manner all of your various units would actually be different physical manifestations/representations of one super daemon… one that could never be truly destroyed because there’s no real part of him in our universe, just bits from our own physical universe that he has twisted and controls.
Although this ‘feels’ original to me, I won’t be the least bit surprised if someone chimes in with “that’s been done before” ;) Seems like it all has! Maybe to make it more original just go with some race that’s never used in the 40K universe and go with the same daemon weapons idea 
~N