I think it's way too early to say that they don't have something envisioned. We have to wait and see the anime first before having we can conclude anything.
The reason why they made it an AU is because they felt R2 was finished and didn't want to touch it anymore. So to make a sequel,they had no choice to make it an AU. Maybe that's why they kept Shirley alive, to clearly define it as AU. Because that's all the movies changed and that's not a lot, if you're making an AU you might as well make a whole bunch of other changes, but they didn't. So it feels they deliberately changed as little as possible, just enough to be it an undeniable AU and that's it.
That way they can expand the franchise and and not change the ending which they felt was in accordance with their sense of ethics and aesthetics.
You have to keep in mind they felt very strongly that Lelouch had to die, they made that clear in interviews and tweets.
I'll dig them up.
Here they call Lelouch's final death their sense of aesthetics, (Continue vol 42, October 2008):
Continue: "Why were you so bold as to choose this ending when the viewers might see it as a Bad End?"
Okouchi: "Bold... yes, we were so bold as to chose this ending. Perhaps the show that is Code Geass ending up this way was decided the moment Director Taniguchi and I teamed up. I suppose you can call it our sense of aesthetics, or perhaps a part of our psychological makeup."
Here they call Lelouch's death a matter of ethics (tweets from May 2018):
<Geass Memories 78> So his end can't be one that forgives him his sins. At least he's aware of his sins and pays for them with his death. This is mine and Taniguchi-director's sense of ethics in our works.
So they feel very strongly about his death, and didn't want to change that in the original series, that's why they created an AU where he will be resurrected in the sequel "Lelouch of the Resurrection".