Mostly we need to remeber to have our bases covered.
We need to be able to deal with flyers, Super heavy's/titans, massed troops, sudden ambushes, flank marching armys, Heavy Infantry, and toughness 7+ monserous creatures with 3+ or better armor saves. Ohh and be able to take and hold objectives useing troops.
Flyers are tough to hit and hard to really hurt without ap 1 weapons luckly they usualy have armor no better than 10 things with AA mounts are ideal and you should remember all pintle mounted weapons are considered to be AA mounted. the odd hydra flak vehicle is ideal but will be targeted quickly failing at that Pintel mounted storm bolters and heavy stubbers on rhino's and chimmeras are probebly the best for there points.
Super Heavy's and Titans can be tough I would like to see the two sides titans fighting eachother but its more likely they will focuse on nukeing troops. titan hammer squads are also very nice especialy with the new storm sheilds. that also makes them great against monsterous creatures.
Sudden Ambushes/Flank marched armys Also drop poding armys and the new demon codex army. they tend to appear where you least want them at the worst time. in most games they can be tough but with flank march and the less formal set up of mega battles you can see whole armys arrive in one turn right behind you. this takes some dedicated troops both mobile assault and ranged to get to them and nutrilize them before they do too much damage. I would be very suprised if we get to turn 3 without a good size army trying to get behind our lines. Idealy the troops dedicated to this should be fast, assault troops, or mounted, bikes and such, mabye even a few deep strikeing units in reserves to deep strike into our lines when needed.
Heavy infantry both sides will have terminators and similarly tough units they can often shrug off all but the worst attacks and I wont be suprised to see 100+ of them headed at us. as allways plasma weapons are great. these types of units are usualy very slow on there own, takeing out there transportation early and giveing yourself several rounds to gun them down is almost required.
Monsterous creatures cant forget these guys and not just tyranids but chaos demons too. unlike most titans these things tend to be decent up close and far more mobile. Similar to titans these things almost require you take them on with similar creatures and mechines. 12 las cannons shooting at a toughness 9 bio titan will only avarage 4 pre save wounds and with 8-10 wounds itself this could get dicey.
Finaly, Holding and Captureing objectives. We all love mega battles because we get to play around with toys normaly too big for the game but we cant forget the rank and file solders those basic troops that every army needs, in some ways guard are perhaps the best army in the game at holding a location, lots of men lots of guns. for objectives in our deplyment zone or near it bunker stratagems can keep our men safe in other locations vehicles with troops in or behind them stand a far better chance then alone.
I know none of this was useful at all but I should be reading..or asleep so I needed to waste time on something.