Apocalypse.
Aside from GW's "you should buy more models!" gimmick, it's also a good way to relax with some folks and do absolutely ridiculous things on the table. 10" blast templates, first turn assaults, models that are physically larger than the player controlling them... Apocalypse is filled with many absurd things. One thing it is not filled with, however, is balance- anyone who's played it before knows that many of the units, formations, and assets are grossly unbalanced (one way or the other), which is something of par for GW's course.
I am of the opinion that better balance always serves a game well. "Competitive" play (if that can even be said to apply to Apoc) obviously benefits from it, but casual play does as well- no one likes being matched up against something unfair, nor to win because a poorly-designed unit made it too easy. So a rebalancing of Apocalypse could make the game more attractive to players and also make games less contentious when played by strangers (such as at a convention or gaming store.)
The first step, then, to make Apocalypse more playable is to take the basic rules and give them a rundown. The one thing that stands out the most are the Strategic Assets, which vary from utterly worthless to nigh-unstoppable. Tweaking these is a good start on bringing balance to the game and relatively easy to fix.
