I believe it is simply not possible taking into account the relatively tight 24 month development cycle to include weather AND night. I find it interesting how you view things with regards to Forza 5, specifically to the lack of content. Allow me to elaborate, it was simply a case of T10 deciding not to port over any content/assets from F4, which itself was built upon F2 and F3. Therefore, F4 was the accumulation of 6 years worth of time energy resources and talent. F5 on the other hand was built from scratch to take advantage of the then next gen hardware and as such had everything built a new.
Moreover it had to deal with a moving hardware target, as they were developing on incomplete SDKs, with no actual X1s, but emulation kits until right at the very end. All this and T10 still managed to give us over 200 next-gen rendered cars, a significantly enhanced physics engine, and the most advanced tire modelling, not to mention next-gen AI in Drivatars.
Forza 6 will be more complete as a package for sure, but until T10 reveal the came at E3, only then will we know what we have in front of us. T10 don't appear to do things by half's, that 60fps update is locked on Forza Motorsport titles, very few games manage this. In fact come to think of it on current-gen Forza 5 is still the only racing title to be released at 1080px60fps locked. In the time frame T10 has I expect more cars, tracks, and night time driving. We may see some wet surface roads. This is just speculation of course, I could be wrong, but lets set our expectation a little more realistically, that way we wont have a reason to rain down on them yet again.