Should Have Died:
1) Makarov - As many above have pointed out, Makarov's encounter (one-sided battle) with Acnologia should have ended with his death. I felt the pseudo-sacrificial storyline (parent staying behind to ensure his kids survive) was written rather well... poignantly beautiful even. Moreover apart from his battle with Jose, Makarov hasn't shown himself to be Wizard Saint level or remotely useful in the subsequent arcs (powerless against Hades, Acnologia, Atlas Flame, Ajeel) so his primary role as a guardian seems like an overstretch now.
2) Gajeel - Once again, I think that it would have been much more convincing if Gajeel's death was the price to pay for defeating Bradman, a Spriggan-level opponent. But then again, it seems that Bradman is back too so I guess... whatever.
Should/Can Die:
1) Elfman - Aside from his romance with Evergreen, I see absolutely no value in keeping Elfman alive. His strength is probably a 60/100 at best. With the existence of numerous male powerhouses such as Natsu, Laxus, Gildars, Gray and Gajeel within the guild, Elfman will almost never get a chance to shine. Furthermore, his magic is of little tactical value nor is it unique compared to Freed (Justu barrier), Bixlow (possession/manipulation) and even Warren (telepathy/communication).
2) Gildarts - If Mashima regrets creating an OP character like Gildarts and thus renders him invisible/away throughout 7/8 of the entire series, then perhaps Gildarts should get one major role and die in the process so that fans would stop pining after him to appear and save the day (that's Natsu's role let's face it).
3) Lisanna - No explanation needed.