I don't know why precisely the Annihilators are headed to Earth this fall. I don't know why they'll fight the Avengers. Obviously there will be some difference of opinion between the groups, but will the Annihilators show up looking for a fight, or will they come on other business and the Avengers get their backs up about these dadgum aliens (and Quasar) throwing their weight around.
The important question is who will kick Iron Man's butt? Somebody's going to do it, but who gets the honor? It's almost too easy for Gladiator. It is too easy for the Silver Surfer. He blinks, Stark's armor explodes around him. Not that I don't find that an amusing idea, but it's a little too quick. Quasar's would pull too many punches, and I imagine Beta Ray Bill will be busy with Thor.
Ronan would be my choice. Iron Man did try to assassinate the Supreme Intelligence once*, which will not win him any points with Ronan. I always enjoy people with hammers swatting Iron Man around like a golf ball (see the third issue of JMS' Thor). Or driving him into the ground like a tent peg, that's also good.
Cosmo would be fine too. I know he didn't accompany the team on their trip to Galador, but maybe a trip back to Earth will be more enticing.
* He may have only argued in favor of it, but expected someone else (the Black Knight?) to do it. Because why get your armored mitts dirty if you can pin the actual regicide/deicide on someone else?