I don't think playing as the spire should be cut. From my understanding, the spire race would just be different, stronger units with more AI response to compensate. In my mind, it seems like that would share 99% of the code with the basic human faction, so the coding shouldn't be a time sink; the graphics would all be different, but in the KS campaign, Chris wrote "Design and art get done before programming, which gets done before balance and QA." I don't know if that's still true with the reorganization, but if it is, the graphics wouldn't be a bottleneck either. That leaves the balance testing, which seems like it should be pretty simple and straightforward - once again, with my current understanding, it would just be changing stats in XML, which could even be done by the alpha and beta testers. All that for one of the main tangible improvements over AIWC that I have seen a significant number of people on the KS express interest in, it seems like it should stay.
Of course, something has to be cut. I would just suggest considering the cost to add in as well as the benefit to add in. But even that is difficult to determine. For instance, I was thinking that the solar systems thing could be dropped, but when looking at the KS page that lists how far along in development different parts are, it says that is already in progress. In fact, looking at everything listed there, it seems that most of the candy ideas that were new to AIWII are already partly coded, while the core things like UI improvements are only "designed", which I take to mean 'we haven't started doing this yet'.
TLDR: rock < Arcen < hard place, as usual