I think the next logical step after SPECTRE was always going to be to deliver SPECTRE II - BLOFELD STRIKES BACK. But if we think a bit about it it's so predictable you would rather head in a different direction, especially since the books already went that way and the series tries hard to avoid the too obvious.
Also sometime early on it seems the prevailing air around SPECTRE turned into farewell rather than unfinished business. At any rate the thread could have been picked up again or left alone for a time; either version seems possible.
Scripts? Eon must have countless of them, most obviously discarded earlier versions of the films we know. Whether 24 and 25 as a two part version was actually in an advanced stage we don't know (yet). But it's just as possible we've actually already got the better part of that supposed second part with SPECTRE. It's the longest film and there was doubtlessly some of it left behind on the cutting room floor. One could imagine a more solid first half, expanded with Bellucci's part and maybe Leiter, ending more or less with Bond meeting a still mysterious Swann who is abducted in his presence. Bond being left behind bleeding in the snow. The whole nonsensical Eyes subplot could then have started BOND 25.
At any rate there will be actual substantive news regarding the future of BOND 25 if it's going to happen anytime soon.
All this is true, but my point is that if a 2018 release is announced, then they likely already had the story, perhaps even a script ready now.
As for the avoiding the the novels because they're predictable... FRWL and CR suggest there's some benefits in sticking more closely to Fleming.
I think Spectre's final scene was pretty clever, in terms of Craig's fickleness. Should he not return his Bond retired gracefully. Should he return they have the perfect set up for 25's inciting incident: the assassination of Swann. This is moving the final scene of Fleming's OHMSS to the first scene of Fleming's YOLT.
In OHMSS Tracey's death is a mistake - Bond the intended target. But in this iteration Blofeld prefers to torture Bond, rather than kill him, so i'm going for Swann being the intended target.