a bit much but here it goes...
i'd retcon the last couple of years of marvel...
i'd have it that house of m never actually ended... that the heroes' apparent victory was just a ruse, and that scarlet witch was still altering reality, directly influencing tony to start civil war...
why? because annihilation never ended, annihilus had taken most of the known galaxy, and wanted earth... but he knew he'd have trouble taking it if every hero was there to stand up against him... his first attempt, house of m, would have robbed the earth of the avengers and f4, but that didn't succeed, so then he tried civil war, which met with more success...
galactus realises that something is very wrong, and he and his heralds (surfer, stardust, terrax), and nova, go to earth to warn everyone... at first sceptical, tony manages to get his hands on wanda, and she confirms everything once doctor strange brings her out of annihilus' control... so tony goes to the raft (the shield helicarrier thingy), and makes peace with captain america (who is actually alive), telling him what's happened...
hulk arrives back on earth, and he and the illuminati forge a truce, to fight off the annihilation wave, and then deal with their problems...
namor brings an atlantean army to the surface to help, and the heroes, who are now back together as one force, prepare for battle...
so basically you have the f4, the x-men (pretty much all of them, x-factor and excalibur too), anyone who's ever been an avenger, anyone else who's a hero, namor and his army, and the inhumans waiting to meet the oncoming onslaught of the annihilation wave, while galactus, his heralds, sentry and nova wait in orbit as the first line of defence...
soon annihilus arrives, and even though the heralds and galactus manage to slay tenebrous and aegis (spelling for both of em not sure), the annihilation wave hits earth, and the planet becomes a warzone (the planet not being destroyed like others, annihilus wants it as a trophy).
the battle rages on, and even though each hero gives as good as they get, annihilus with his recently acquired quasar powers seems to be gaining the upper hand...
that is until thor arrives with a massive asgardian army...
long story short, thor and his buddies go to town on the annihilation wave, their initial attack letting the heroes regroup and come back at annihilus, and with galactus and his heralds now back in the fight, annihilus' forces are defated...
havent thought of a fitting ending... though i had thought that maybe tony would be the one to finish it off, as he blamed annihilus for making him start civil war...
for anyone wondering, thanos hadn't actually come to earth, he'd simply sat back and waited to see what would happen if these two massive forces ever met, which they did, so he honestly just wasn't bothered getting involved...