I was assuming only allied pairs in BFZ; I don't know whether that's known or not. It is true that, especially if enemy-colour ones exist, they actually offer Scapeshift more flexibility inasmuch as "ten mountains" no longer has an obvious best configuration of eight shocks and two basics but can instead contain various ratios of blue to green. The most useful form of this would be cutting basics and/or RG lands for UR lands, as you suggest, so this doesn't matter too much if there are only ally ones. (Though actually, it's not immediately obvious to me how exactly to break down the six non-Steam Vents mountains between Stomping Ground, the new RG land, and basics.)
A little piece of evidence I noticed for these lands (or at least, lands with basic types) being real: Nissa, Vastwood Seer specifies "basic Forest". Most cards that search up lands of a particular type don't have that rider, and I doubt that Wizards was afraid of her taking over Modern/Legacy without it. It seems pretty likely that it wouldn't be there unless it was supposed to matter in Standard.
Even if fetchable, these are effectively just CIPT lands. By the time they start coming in untapped you don't desperately need it; they are bad for the very same reason the fastlands are so good, and that would be true even without the word "basic" which makes them much worse.
The issue is that Modern has really good mana. Most decks aren't running a million shocklands and dying from the pain; they're running as few as they can get away with while still being able to fetch what they need. I don't think it's ever correct to cut your last shockland for one of these because sometimes you really need to fetch untapped early, so decks that are only running one of each shock (which is pretty common) just aren't interested.
That said, there may be a few decks which are interested. For instance, the WUx decks that run Hallowed Fountain because it's fetchable but really really hate taking pain from it in most matchups. Maybe it's correct to go down to say, one copy of Fountain (for those cases where you really need to fetch an untapped land early) and replace additional copies with this new thing. Then again, those decks still want untapped lands on the first two turns. I think Scapeshift actually does cut some of its Stomping Grounds for the RG one, since it's locked into running lands with basic types but tends to wish its manabase were less painful (and with all those Sakura-Tribe Elders and stuff, you get to two basics in play pretty consistently). Neither of these decks are not exactly dominating the format right now, though.
They really shouldn't since the decks that actually use jace (devle decks) will cause him to activate off of his first tap.
And what if you want to loot twice anyway?
I mean it might be wrong to play Minamo, but whenever I see a deck that could use a Kamigawa legendary land but isn't playing one I wonder whether someone actually decided it wasn't worth it or if they just forgot that they exist.
If you're really scared of Choke, you already have the ability to go up the Sulfur Falls count, add a couple of Cascade Bluffs and replace some of your basic islands with Oboro, Palace in the Clouds and Minamo, School at Water's Edge. People don't do that because Choke doesn't see enough play and because Twin wants to be Moon-resistant so it can play Moon in its own board. I don't think the equation changes by adding a fastland; right now manabases are being built to have fewer nonbasics, not more, and I don't think that would change just because the nonbasics got slightly better.
A UR fastland would probably replace all or most copies of Sulfur Falls, but I don't think it otherwise changes things much. In theory you could run both, but your mana is already good enough that I don't think you want to make yourself weaker to Moon that way.
I don't think a UR manland takes Twin over the top at all. Grixis Twin doesn't usually play Creeping Tar Pit, and I don't think it's immediately obvious that UR Twin is better suited for one. (I mean if you desperately wanted you could even play Faerie Conclave and nobody is doing that.) In particular, Twin is a deck that wants to keep mana up on the opponent's turn, and also doesn't play humongous amounts of land. If you printed Celestial Colonnade with the white symbols shifted to red, it would be pretty bad in Twin because it would eat up too much mana on your own turn to actually use. There is a reason that the decks that play Colonnade now are playing three more lands than Twin does.
I've run a Temple of Mystery in Scapeshift before. They come up here and there, but you need a few things to go right in order to use them. Most Modern decks just do not want CIPT lands. Manlands may be good enough to run anyway, but anything less than that isn't. And even in a situation where you don't mind a CIPT land, you'd still usually play a manland over a scryland when one is available, so that eliminates the allied ones (except maybe BR) from consideration in most decks. An all-in combo deck like Ad Nauseum is the exception that would actually rather have a scryland than a manland.
Some Twin lists still play a copy or two, but as the deck evolved from an all-in combo deck into a control deck with a combo in it, the need for redundant combo pieces was lessened. They usually don't play the full eight 2U guys anymore either. As far as Pod goes, the Melira version of the deck was ultimately judged better and the Kiki deck faded. And then, of course, Pod got banned. Collected Company is the "replacement", but you need your combo pieces to cost three or less for that. RWU Kiki Control was a thing for a while, but right now RWU of any flavour is not great.
So basically he just doesn't have a real home right now. At some point he may see some play again, but he costs five mana and dies to Bolt. I wouldn't expect him to take over the format.
Buried Ruin was printed in M12 and based on the fact that Goblin Fireslinger is in, M12 looks to be am eligible set. This is not to say that Buried Ruin will necessarily be included, but it is possible.
Oh, I'd missed that about M12. I guess it makes sense since M12 rotated with Scars block. Buried Ruin is a reasonable possibility, then. I still think it's less likely than Boseiju/City/Citadel though.
Boseiju sees Modern play and hasn't been reprinted outside an FTV. It seems like an ideal candidate for the set.
What does everyone think is the last two land slots? I'm betting Cavern of Souls and Encroaching Wastes.
The first one is very likely either City of Brass or Buried Ruin, and the second is almost sure to be Evolving Wilds.
Buried Ruin is ineligible. My guess is Boseiju, but CoB and Citadel are also plausible. (Desert and Contested War Zone are the other possibilities that aren't part of cycles. Neither seems completely impossible, but they're less likely.)
how important is it to run 23 lands? I wanted to make a twin deck but keep ending up with 61 cards maindeck, so anyone tested 22 lands?
Dickmann's Spreading Seas list has 22 lands. I'm playing a variant on that list and it's worked okay but I've definitely wished I were playing 23 at times.
Junk is popular, but it's at most 20% of the meta and while it's a bad matchup it's far from unwinnable. Twin is favoured against most of the other decks in the format.
The issue is that Modern has really good mana. Most decks aren't running a million shocklands and dying from the pain; they're running as few as they can get away with while still being able to fetch what they need. I don't think it's ever correct to cut your last shockland for one of these because sometimes you really need to fetch untapped early, so decks that are only running one of each shock (which is pretty common) just aren't interested.
That said, there may be a few decks which are interested. For instance, the WUx decks that run Hallowed Fountain because it's fetchable but really really hate taking pain from it in most matchups. Maybe it's correct to go down to say, one copy of Fountain (for those cases where you really need to fetch an untapped land early) and replace additional copies with this new thing. Then again, those decks still want untapped lands on the first two turns. I think Scapeshift actually does cut some of its Stomping Grounds for the RG one, since it's locked into running lands with basic types but tends to wish its manabase were less painful (and with all those Sakura-Tribe Elders and stuff, you get to two basics in play pretty consistently). Neither of these decks are not exactly dominating the format right now, though.
I mean it might be wrong to play Minamo, but whenever I see a deck that could use a Kamigawa legendary land but isn't playing one I wonder whether someone actually decided it wasn't worth it or if they just forgot that they exist.
So basically he just doesn't have a real home right now. At some point he may see some play again, but he costs five mana and dies to Bolt. I wouldn't expect him to take over the format.
Boseiju sees Modern play and hasn't been reprinted outside an FTV. It seems like an ideal candidate for the set.
I agree the other slot is Evolving Wilds, though it could also be Exotic Orchard.
Duress is the only one that sees any Modern play at all.