MyrHerder
Well, I can't say anything for the Halo 4 ODST and H5 Nightfall other than being fan-service and just to get more REQs. True ODST Mjolnir is the Helljumper variant.
Well, we never seen the Nightfall armor earlier, only in 2556 - by Locke and his squad.
I could think the ODSTs in Amber Clad (for now, unless 343i changes the look of the ODSTs in Halo 3 Anniversary,etc.) had more experimental armor, the newer generation, while all other ODSTs we know, had the normal one.
Analyzing further, we had a picture in Halo 5's campaign, Hunt The Truth:
Still, if I look at such screenshot from the trailer, I can't help it but feel they made the chest and shoulder plates bit bulkier:
ALSO, here is a screenshot of the ODSTs in Halo Wars 2. As you can see, they have the normal ODST chest plates... So, we might just see two generations - just like the Bungie Pelican and 34i3 Pelican.
For now I think the old Bungie ODSTs are from the Spirit of Fire, while the new 343i ODSTs are from the UNSC research bases.
While I like some parts in 343i's design, I still prefer some parts from Bungie's design - like the helmet, chest, etc.
foxleader - they were drifting in space for 30 years... we don't know what happened during these 30 years and we still don't know how did they get to the Ark. The trailer and current info didn't explain anything.
There is also no explanation as in 2558 when Spirit of Fire was still drifting on course to an unknown planet (which might be the one where the Banished reside), there was an alarm to abandon ship and a Flood infection form.
Yet, Halo Wars 2 takes place in 2559 - so where is that Flood, etc.