The whitebeard war saga was my personal favorite story arc, and was extremly epic. Oda's drawings of all out war, the power of the admirals, Shichibukai, sengoku, the blackbeard pirates, Doflamingos speech, the death of Ace, and shanks stopping the war was a literary masterpiece.
However, i was very sad that ace died. Luffy tried so hard to rescue him, but failed. he had lots of help of corse, but who he didnt have help from was his crew. I dont think it would have made a differnce before the 2 year skip, but afterwards, i think that they could have saved ace.
The way i See it
Luffy: Fully trained in haki, can go toe to toe with and admiral now.
Zolo: Trained under Mihawk, he's one of the worlds greatestswordsman
Nami, the perfect clima tact, plus her new knowledge of weather, could have powerful affects against the entire battlefield.
Usopp: Arguably the world's best sniper, except maybe his father, or Van Augur. Kabuto is a powerful weapon, and would allow him to strtegically attack anyone in sight.
Sanji, after trainng in New Kama land, he is an extremely powerful fighter.
Chopper: He has researched mroe on his devil fruit ability, and should have plenty of new abilities.
Nico Robin: was with the revolutionary army, and there is no way you can go wrong with that.
Franky: Should be as powerful as a pacifista, with the versatile mind of a human,and the craziness that is franky, nuff said.
Brook: He has probably mastered new forms of musical powers.
So, do you think they could have made a difference?