JR gives some spoilers while remaining iffy on many points. Things aren't really clear, in some interviews he said 'x' may or may not happen, while in a different interview his statement sounds more like 'x' will happen. But from what I've gathered:
The 6 year time jump sticks but there will be flashbacks. Flashbacks are likely for all three groups (bunker, ark, and Clarke & Madi).
The six years are significantly longer than the rest of the series combined (which was 5-8 months), so "you can expect real changes" with the characters.
Madi, the Nightblood child:
Clarke and Madi have a "mother/daughter relationship", Madi is "her surrogate child essentially".
"Clarke was alone until she found Madi" and "they have been together already for a number of years".
One interview says "she's been with her for five years" (so found her about a year after Praimfaya), while another puts their meeting later: "found each other at some point a few years into being the last person the planet".
Clarke has been with Madi longer than her entire time with Bellamy & others (~5 years vs only a few months), making their bond very strong.
JR said multiple times that this relationship will be important in season 5: "really important relationship for Clarke in season 5", "a huge part of season five", "it's going to be a very, very important relationship in Season 5".
"Clarke will now have to face some choices that put Madi on one side and her friends and her people on the other." "We'll see that tested... in Season 5."
"When we see Octavia at the beginning of season 5, it's a pretty big 'WTF' moment".
Octavia will have a brand new look.
Octavia may have led in ways Bellamy wouldn't approve.
They had their own share of hardships.
Implied that Bellamy was the leader on the Ark.
Child(ren) may have been born on the Ark.
They're not on the prison ship... or they might be? – "see who is... on that ship, but it is not Bellamy and Raven... so unless they found another way... which I suppose is a possibility, that's not who we're going to find in that ship."
"It will for sure be a threat", "clearly is going to be a problem moving forward", "the prisoners are going to start as antagonists".
They're criminals, like the 100 were, but they're "real hardened criminals" ... "perhaps".
Ship signs include Eligius Corporation and Gagarin Prison Transport – "These names are not randomly selected."
"[They're] coming back to Earth thinking that its abandoned and that its their planet".
Clarke has been "journeying" and "scavenging".
"We will tell the story of how that valley survives."
Other Nightbloods not in direct path of the death wave, may have also survived.
We should expect at least one more child: "there probably will be another young character who play a role next season".
The only thing that irritates me in this is that there might be kids on the ark. I know that our protagonists are 24-30 by now, but i just can't put that in my head, neither could i imagine that e.g. Bellamy and Raven could have kids together O.o
Kid wise, its entirely possible that harper and raven still have contraceptive implants. Raven def had 1, being over 18, and was unlikely to remove it. Harper might have had one, i dont know the age they get implanted but it's possible that at 16 or seomthing it happens, bc the delinquents are not new tomsex and population control is more important than teaching abstinence in the Ark.
That leaves emori and echo. IF there is a child, emori and murphy have a lot of sex, so thats a gkood guess. They are also committed as of S4 Praimfaya. Not sure bellamynwould get with echo right away, but it has been long enough, we really have no idea who hes sleeping with 6 yrs later.