The block after Innistrad 2 will be about a new plane called Sivagat. It's a desert world and will heavily reference Egyptian mythology. It will feature gods and the story will be a relatively small-scale mystery plot that's gonna be completely separate from any other story line going on at the moment. The reason for this separation is that WotC is worried that the block will mirror Kamigawa in certain regards due to the exotic setting and almost niche mythology that's not as popular as Greek mythology.
The first set of the block will be called Secrets of Sivagat and the second set will be called Fury of the Sand.
The block after that will be another Return to Ravnica. I have no information about that story and only know the name of the first set. War of the Guilds.

Maro says there will be an announcement on Monday - probably the next duel decks?

Maro says there will be an announcement on Monday - probably the next duel decks?

Ravnica again would be cool, but yeah I have no reason to treat this as credible. I'm particularly suspicious about getting another block with gods.

1. Converted mana cost

Remember when I said that the characteristics of the face that's up are all that matter? That wasn't quite accurate. Under the new rules, the converted mana cost of the back face of a DFC is based on the mana cost of the front face. (Previously, because the back faces lacked mana costs, their converted mana costs were all 0.) The one exception is if something is a copy of the back face of a DFC, its converted mana cost is 0. So If I control Insidious Mist, it has no mana cost, but its converted mana cost is 4. If I then put a copy of Insidious Mist onto the battlefield, that copy's converted mana cost is 0.

2. Entering the battlefield transformed

A few effects can put a card onto the battlefield transformed. This means to put it on the battlefield with its back face up. Starting with this set, we have a new rule that if you're told to put a card that isn't a DFC onto the battlefield transformed, it just stays where it is. This may affect a few cards in this set, but I think the Magic Origins planeswalkers illustrate the change nicely. (This is code for "The cool cards in this set I could use to illustrate this haven't been previewed yet." I know. I feel the same way.) Say you control a Clone that's copying Jace, Vryn's Prodigy, and you activate faux-Jace's ability with four thousand cards in your graveyard. Well, four thousand is "five or more," so faux-Jace dutifully exiles himself. Then he tries to return transformed, but in exile he's just a single-faced Clone. He's not a DFC, so he gets detained at the border and remains in exile. I hope the card you drew was good, because this wasn't a great play otherwise.

3. No flippy-floppy

I've saved our most technical change for last. Here's the new rule: if a DFC has an activated or triggered ability that transforms it, that permanent transforms only if it hasn't since that ability was put on the stack. What does that mean? For example, say you activate Elusive Tormentor's ability, then you activate it again in response. Why? Because you want to discard a lot of cards. Why? We're almost there. Relax. The first ability to resolve will cause Elusive Tormentor to transform, as expected. But the second ability won't, as Elusive Tormentor has already transformed because of the first ability.

some additional speculation on the walkers of SoI block.

Nahiri: possibly more wanting her to get a standard printing than evidence but ehh. Innistrad is known for its use of makeshift weaponry, which would work well with Nahiri's mechanical focus on equipment. She's also confirmed to have visited the plane in the card Declaration in Stone.

Jace: I've been unsure if the printed walker would be Tamiyo or Jace for SoI, until I saw the packaging. Jace is plastered all over this set and a focus of the pre-release. It's also worth noting that Tamiyo seems to be off of Innistrad during the events of the block, as the flavor text seems to be referring to her as if she's dead or missing.

Sorin: Possibly a return to mono black, this is my weakest guess and mostly derived from the ending of Oath's story and the fact that it's Innistrad. I personally don't want another Sorin, especially a WB one, because then Abzan tokens becomes 100% legit.

Tibalt: This one seems obvious to me, Avacyn's been shown now to have not become evil, but to have gone mad. She's in a blood frenzy and killing indiscriminately. Her flip card having her become red and inflicting damage to everything is incredibly Tibalt themed. Tibalt's specialty is pain and he does have a grudge against Sorin.

Arlinn: Because you don't introduce a new planeswalker to the story without a card.

The prerelease boxes are apparently in the image of Tamiyo's journal, though. I was sure she was dead because jace but with her making an appearance it would seem weird to bring her back only to not card her.

The planeswalkers in SOI will almost certainly be Jace, Sorin, and Arlinn Kord, since they appear on the booster packaging.

Liliana is the posterwalker for EMN, so she'll probably get a card. Unless they go over the normal 5 planeswalker quota, that leaves one slot left for either Nahiri or Tamiyo.

I won't say it's impossible that they pick Tamiyo over Nahiri, but the way the story is going, I don't personally think it's likely.

This. Also, having Tamiyo in the story without a card at least acknowledges that she's alive and can be used in a future block. That's a lot more than Tibalt fans seem to be getting.