Here is my concoction:
1) Start with any ol' D&D-esque ruleset
Since B/X is my favorite let's start with it.
2) Add some supplementary rules material. You're primarily looking for new Gygaxian building blocks (classes, races, spells, monsters, magic items, etc) to drop into the game. In this recipe you want exactly two different sources for this stuff, one of which is easy to put into your game...
I have been reading Mutant Future again while thinking about a potential Thundarr-esque one shot so even though Jeff mentions it in his post I will stick with it as well.
...For the other one choose something that might be a little harder to fit into your system of choice without some work.
The other game I have been reading lately has been Barbarians of Lemuria so let's choose that.
3) Now you need some fluff to hang all this stuff on. Pick exactly three sources of campaign inspiration. Two of these sources should be recognizable as fantasy material...
First, The Secret of Sinharat by Leigh Brackett.
From Paizo's website:
Enter Eric John Stark, adventurer, rebel, wildman. Raised on the sun-soaked, savage world of Mercury, Stark lives among the people of the civilized solar system, but his veneer of calm masks a warrior’s spirit. In the murderous Martian Drylands the greatest criminals in the galaxy hatch a conspiracy of red revolution. Stark’s involvement leads to the forgotten ruins of the Martian Low Canals, an unlikely romance, and a secret so potent it could shake the Red Planet to its core.
Second, The Misenchanted Sword by Lawrence Watt-Evans.
Ethshar and the Northern Empire have been at war for hundreds of years. Hardly anyone alive remembers why, or over what. The tempest, turmoil, and war are endless, and the killing more endless still. The war has become not just a way of life, but an institution; no one dares to dream that it could end.
Not even Valder of Kardoret, Ethshartic Scout, trapped behind enemy lines.
A MISENCHANTED SWORD
But now everything has changed: at a moment of great need, a hermit wizard crafted Valder a magic sword called Wirikidor — a blade at once cursed and enchanted, a misenchanted blade that makes him unbeatable.
Your third fluff is meant to be the wild card. Pick something way out in la-la land for this one.
Way out in la-la land, eh? How about... the 1978 pilot episode of Battlestar Galactica.
So for rules we have:
Barbarians of Lemuria
And for fluff we have:
The Secret of Sinharat
The Misenchanted Sword
and Battlestar Galactica.
Take rules light D&D with wizard/warrior elves, dinosaurs and dragons, add a dash of physical and mental mutations and missile silos. Mix with traits and flaws and evil druids that worship the 20 dark gods of the void.
Put it all on top of a dying mars with a war going on for so long that no one remembers why they are fighting and add in cylons... its gotta have cylons.