Orks don't have enough material to make a whole game out of? Come on you give them far too little credit. They got all kinds fo potential classes (Mekboys, Weirdboys, Flashgitz, Nobs, Stormboyz, etc.), all kinds of different flavors of clan (like Chapters). You could EASILY flesh out a game as large as any other core book, as Ork society is a hoot and a half on it's own.
Each core book has a "gimmick" on top of the core rules, such as the way Squads work in Deathwatch. What I envision for a rag-tag team of mismatched Orks is a heavy emphasis on vehicular-based combat. Not big ol voidships like in Rogue Trader, but the Mad Max-ian antics of something straight out of Gorka Morka.
The flavor of the fluff is that your band of gitz are pirates and bandits roving the land for teef and flashier dakka, sometimes tis means going to other worlds or smashing aboard an enemy voidship and just mucking up the place. On top of everything it would ramp up the dark humor that all the previous games have had (at least the games I've played) into fun romp. Oh, and there must be random tables GALORE for everything.
But, barring that. I really would like a "WHRP40k" Compendium that puts a final stamp on the ruleset. I wouldn't mind rebuying second editions of all the games if they were reprinted as splatbooks for the main, unified ruleset.