great hypothetical question.
personally i wouldnt want to see another strawhat that would die, since merry already died and all of the crewmembers had promised to their loved ones of returning after fulfilling their dreams.
if there would be a strawhat ill choose to die, i'll choose ussop as many guys here mentioned, firstly, it would be super-emo because he and luffy are almost like brothers, they fought together and quarrelled and yeah., ussop's dream is indefinite.
im thinking of a scenario where the strawhats need to escape and then ussop thinking of how unreliable he was, he would suck up all his courage and help the crew escape while he fends off the pursuers and dies in the process, shouting "i got no regrets!!! im a brave man of the sea father!!!!"
or after meeting yassop, he goes into duel against a mighty opponent of the crew at the end of the series and got fatally wounded but killed his opponent as well,…he would be offered treatment but would refuse since he felt like he achieved his dream and will order luffy to fulfill his dream as well.
if the thread-starter may excuse me, if i can give a death in one piece, i'll choose mihawk and shanks. im thinking of a scenario like, luffy before reaching raftel will have a friendly duel against shanks, and then while luffy goes on to reach raftel, blackbeard is on their tail and shanks intercepts them....shanks and his crew gets beaten and shanks would get killed by blackbeard,...now luffy will avenge shanks and returns the hat. i think that would be pretty heart-wrenching.
and shiryuu at some point by the end of the series kills mihawk and steals the dream from zorro, so zorro needs to beat shiryuu to avenge mihawk, his mentor and be the greatest swordsman alive.
it would also help the pirates' side to be not too overpowered when the final battle ensues. shanks and mihawk already appeared in the war. i think in the final battle they will not be there, because its gonna be monkey d. dragon and luffy who would be the main alliance there.