Before reading this topic, somehow it had not even occurred to me that Gendo might inject the Key of Neb in Final, even though I already knew about the syringe.
Now I'm convinced that's exactly what's going to happen.
But not just because of this topic.
I'm sure I've mentioned this before, but my reaction to Q was far different from that of I think most of the audience in the sense that I didn't interpret it as going in a wildly different direction at all, in fact I was more than a little disappointed that it was essentially just a repeat of the heart of episode 24. The major beats are all there: Shinji has become alienated from Asuka, Rei, and Misato, then finds Kaworu only to lose him and wind up even worse off. The only thing Q really changed was to crank the contrast on those basics up hard enough to finally make them stand out, so hard Shinji's feelings seep through the screen and into the audience ourselves. The world becoming ruined, Misato et al's changes... It's all really designed to put us in Shinji's shoes, getting us to empathize with where Shinji was by this point in NGE better than NGE could (at the cost, in my opinion, of a lot of what kept my emotional interest in NGE, but who knows yet if the payoff will be worth it?).
So the reason I would expect Gendo to inject the Key of Neb is not just because the Key of Neb happens to be a syringe, but also because it is Rebuild's obvious parallel to the Adam embryo, simplified to the essence of its role, and after Q proved to me that we should still expect to see a lot of things that remind us of what we've seen before, that leaves me expecting at least a little of the same in Final.
That said, I don't expect Final to be as much of a retread of EoE as Q was of ep 24. Q winding up being such a retread reminds me of something Anno (I think) said at the beginning of the project: that it would be essentially a retelling of NGE but ending with a brand new conclusion. With Ha we all assumed this original plan had been discarded, but in the new perception I have that was given to me by Q, we have (to me) still been watching little more than a retelling of the same basic story as NGE, hitting the same beats. So if that's the case, Final will be where it diverges "for real".