This is something I have been thinking about for a long time. If I were to redesign how 40K battles were fought, and I could only change one major thing, then the proposal below would be my first choice:
—-BATTLE GROUP 40K PROPOSAL—-
*When playing a game of 40K in a “competitive” setting (i.e. tournaments, competitive games between friends, etc.), then each player would use an army consisting of (1) PRIMARY BATTLE GROUP and (3) SUPPORT BATTLE GROUPs.
*The PRIMARY BATTLE GROUP would be made up of 75% of the overall points value of the game (i.e. so, in a 2000 point game, the PRIMARY BATTLE GROUP would be 1500 points) and would be created using any Battle-forged combinations of detachments and/or formations, so long as it was no more than 75% of the total points value that the battle is being fought at. The PRIMARY BATTLE GROUP could draw units from up to two different factions and must contain your Warlord.
*The three SUPPORT BATTLE GROUPs would each be made up of 25% of the overall points value of the game (so, in a 2000 point game, each separate SUPPORT BATTLE GROUP would be 500 points strong) and, like the PRIMARY BATTLE GROUP, would be created using any Battle-forged combinations of detachments and/or formations, so long as each individual SUPPORT BATTLE GROUP was no more than 25% of the total points value that they battle is being fought at. Each individual SUPPORT BATTLE GROUP could draw units from a single faction (but each of the three SUPPORT BATTLE GROUPs could be drawn from different factions that one another and/or from the PRIMARY BATTLE GROUP).
*Before each player deploys (i.e. after rolling for table sides, mission type, and who deploys first, but before each side actually begins rolling for WL traits, Psychic powers, or begins deploying their forces), there would be a 5 minute time period set aside where both players would be given the army list of their opponent’s PRIMARY BATTLE GROUP. During that time, both players could study their opponent’s list and then choose which (1) of his three SUPPORT BATTLE GROUPs they will choose to take for this particular game, based on which if these SUPPORT BATTLE GROUPs will help them the most against the particular army they will be facing (Note: Neither side will be able to see the lists for their opponent’s three SUPPORT BATTLE GROUPs, so their decisions will be made solely based on what is in their opponent’s PRIMARY BATTLE GROUP list). Once the 5 minutes is up and both players have decided on which SUPPORT BATTLE GROUP to take in the current game, then the game continues just like it does in the current rulebook, with Deployment, Infiltration, Scout moves, Seize the Initiative, etc. and all the other parts of a “normal” 40K game.
*Finally, if PRIMARY BATTLE GROUP is made up of entirely of units from the same faction, then you can choose to modify your roll on the Warlord Trait Table by +1/-1 if you choose , after any re-rolls that you might get (Note: rolls of 6 could become 5s or 1s, and rolls of 1 could become 2s or 6s). Additionally, if your ENTIRE ARMY (i.e. both the PRIMARY BATTLE GROUP and the SUPPORT BATTLE GROUP you selected for this game) are made up of entirely of units from the same faction, then you can modify any Reserve rolls you make by +1/-1 if you choose. This would provide a bonus for more “fluffy” armies that are all made up of a single faction, since they are sacrificing the flexibility of being able to take allies from other factions; thus, they would be rewarded with the enhanced WL Trait rolls and/or improved Reserve rolls to represent the cohesiveness of an army from a single faction fighting together as one.
So, why do all this? Why add another level of rules and decision-making to an already complex and challenging game like we have in 40K? Well, there are two big advantages that I think playing like this would bring to competitive 40K gaming:
 The biggest advantage of this playing format is that players can bring larger portions of their army to games and then, based on their experience, tactical acumen, and what they learn about their opponent’s list in that 5 minute pre-game block, “tailor” their army to having the best chance of success. For example, if you take a PRIMARY BATTLE GROUP of Drop Pod heavy Space Wolves and are facing an enemy PRIMARY BATTLE GROUP of Wraithknight heavy Eldar in a particular game, you might choose a SUPPORT BATTLE GROUP that has some strong anti-Monstrous Creature abilities, such as Space Marine Grav weapons, TWC with lots of Power Fists, or Grey Knights with Dreadknights/lots of Force Weapons.
Alternately, if you find your Space Wolves PRIMARY BATTLE GROUP is up against a very Skimmer and Flyer-heavy Dark Eldar PRIMARY BATTLE GROUP, then you might opt to take a SUPPORT BATTLE GROUP that includes a unit of Skyfire capable units like Stalkers or Hydras or Onager Dunecrawlers in it. The beauty of it is that, while your PRIMARY BATTLE GROUP stays the same, from game to game you choose which of your (3) SUPPORT BATTLE GROUPs to take, thus allowing you to tap into much more of the potential abilities of your entire collection and also increasing your chance of having the best units for each opponent you face.
Put another way, say you are in 2000 point tournament; with this playing format, you have 1500 points that are “set” and will be used every game, and then you have another 1500 points from which you can select 500 points worth of models to “reinforce” your main army each game, based on who you face. In many ways, this is like a football team, which may have a base defense, but then has multiple “packages” it can put on the field, based on who it is playing that week or even on what kind of offense their opponent is putting on the field. Thus, you are not “locked” into a single army format for the tournament, based on the list you designed before you even arrived, but have some flexibility to respond to the type of enemy threats you are encountering (I am a infantry officer for my real job and this is what we do for real life combat operations; i.e. match “troop to task”, based on the mission, rather than just blindly send the same task force organization for every mission we are given).
 The second big advantage of using this playing format is that it allows a much larger portion of your 40K collection to potentially come in to play, which I think is outstanding. For example, if you have models that are decent at specific missions (e.g. take the Stalkers AA tanks against flyers) or against a specific opponent (e.g. Grey Knights fighting against Daemons), but are not so strong elsewhere, then they may never (or rarely) get used in competitive play, since you cannot afford to have them in your army list for such situational match-ups. With this playing format, however, you could include them in one of your three SUPPORT BATTLE GROUPs and, if the situation did come up where they make sense to take, you could reinforce you army with them (and if not, well then you keep them in “reserve” and take one the other SUPPORT BATTLE GROUPs).
Personally, I would love to be able to bring multiple supporting parts of my collection to a competitive game, even parts that I normally would not include, and also see other players get a chance to bring parts of their army that normally do not get played, but could be standing by to be brought on the table-top for a specific scenario or match-up. A great example of this is Imperial Assassins… in many scenarios, they are not that competitive and/or a poor match-up, but in other scenarios, being able to field one or more Assassins could make a BIG difference (i.e. if your opponent is fielding an army with lots of Psykers, like 1K Sons, Eldar, Grey Knights, etc. then you could select a SUPPORTING BATTLE GROUP that includes a Callidus Assassin; if your are facing an opponent with no Psykers at all, like Tau or Necrons, then you could leave the Callidus and his SUPPORTING BATTLE GROUP in reserve for that game and choose a different SUPPORTING BATTLE GROUP that is more useful against that opponent). Similarly, if you find out that your are playing a particular mission type, then you could select a SUPPORTING BATTLE GROUP with units that are particularly suited for that mission (i.e. maybe you take a bunch of fast, MSU units if you have lots of Objectives or, alternately, if you are playing a Kill Points game, maybe you take an allied Imperial Knight instead).
Overall, I think this would not only be a very fun way of playing, but it would have the side benefits of, one, encouraging players to really study their own armies and even their opponents armies more in order to improve their ability to decide which SUPPORTING BATTLE GROUP to take each game and, two, it would encourage players to expand their collections even further, since they could gain advantages in competitive gaming by having access to more options (this gives players a reason to delve further into the hobby and also is a boon to GW, since it would likely boost their sales, in turning hopefully encouraging them to continue supporting the game even more aggressively with new rules and models and fluff).