Awesome thread and one that is very close to my heart. So, what do we know for sure:
Ceremony & IALP written and were planned to be recorded on the American tour. Going by previous patterns, this almost certainly would mean they were the follow up single to LWTUA.
We know that they planned at least a Scottish tour for July (I was supposed to go the Glasgow gig and I've from others of an Edinburgh show too). Going from previous patterns again, it's likely they'd have some new material ready for these shows.
I reckon we'd have seen the Ceremony single come out in September/October and a new album recorded over the winter and released in March/April '81. They were writing at such a tremendous speed by the point at which Ian died that they'd easily have a new album's worth of song's by then.
It's clear they were already experimenting with electronics and i think the new album would have seen some small steps in that direction e.g. I'm pretty sure a version of Truth would have emerged in some form or other. The majority of the album however would have been a progression of the second side of Closer/Atmosphere direction - stately, synth focussed atmospheric tunes e.g. Doubts Even Here would have fitted into this quite nicely.
To coincide with the new album, a new single would also be released (non-album of course). I like to think that a Procession type of song would have been this single.
As strange as this may sound, I think that, album wise, they would have quite quickly headed in a similar direction to how Dead Can Dance turned out in their mid to late 80's style. Atmospheric, almost ambient, songs written using an electronic backbone but with acoustic instruments on top - like New order but with the poppy, lightness replaced by a continuation of the JD darkness.
For singles, they would have continued to release their faster, lighter songs e.g Procession, maybe a variant of Dreams Never End, possibly a toned down version of Temptation but I don't ever see them going as mainstream pop/dance as New Order did.