I'd like to Sanctus be young during his fight with Nero.
.........Not sure why but I actually kinda was fine with his appearance but then it would've been epic if he did turned into a more fleshed out version of the Savior.....actually the thing that disappointed me most was the fact the final boss fight was so simple.
I would've liked it to be like you fight Sanctus in his original Diabolical form & then in the second boss fight he DT's like how Arkham(using Sparda & energy from the Savior) did but instead turns into a more fleshed out version of the Savior & younger too with a younger voice(heck Liam O'Brian can easily shift from the old guy voice to the young badass like Caius from FFXIII-2 or an evil Gaara from Naruto).
If not that turn the final event boss fight(Nero VS False Savior) into an ACTUAL boss fight not 2 timed Buster counters & end.