I think what frustrated fans when Aerith died in the OG other than they thought she was the love interest, was all those possibilities that suddenly close. It's true too in Rebirth: ToA is the place where Aerith starts to finally accept her Cetra lineage, in the OG you can get her best weapon right before, etc. Everything seems to line up for her (big red flag when you think about it), and then she dies.
And it's also a fact that we know Aerith was most certainly doing things behind everyone's back in the LS; but in the OG they managed to not show her again, which is good enough for Nomura, because this time around, they're certainly going to show us what she's going to do in the LS. Aerith is unique in that she's the only character (with Zack, because Zack is protected by her/the Planet) whom they can still use once dead. That's also one of the reasons why Tifa was never going to die.
I've said it pre-Remake, but even back then I thought we were going to see what Aerith does in the LS, and that's why Nomura says that her role isn't finished. Because this time, we're going to see how she saves the Planet - it is her role after all.