Masterpieces have driven the standard staple prices down a lot these past few sets anyways. Those cracking packs for value will continue to do solely in the hunt for those lotto tickets, not the rare valuable in-set card.
On a related note, I think it'd be a good idea to go over the modern/eternal staples that will be rotating this fall. While it's probably not the best time to buy, it's still worth being mindful of the cards from Soi and Zen blocks that are going to be leaving the card pool in case you want to go ahead and secure your playset for larger formats.
BFZ:
Gideon, Ally of Zendikar- Shows up as a 1 - 2x in various DnT builds for the control matchup. Also probably has some value in the modern RW prison decks.
Cinder Glade- Unlikely to ever go high value, but it is worth noting that this is the only battleland, to my knowledge, that sees older format play. It's a standard include in Valakut decks.
Bring to Light- On the Valakut note, B2L has also seen play in some scapeshift variants in modern. Those have been on the downturn lately, though, so I don't expect this to ever be super pricey.
Ulamog, the Ceaseless Hunger- Common tron target and casual favorite. Has already gained quite a bit of value, although maybe the recent Marvel banning will create a nice gap for those looking to pick him up for other formats before he goes back up again.
Shambling Vent- The only manland from the block likely to continue to see play.
Oath of the Gatewatch:
Matter Reshaper,
Thought-Knot Seer,
Reality Smasher,
Endbringer,
Eldrazi Displacer,
World Breaker- All of the rare, constructed playable eldrazi already seen more play in modern and legacy than they do standard, and I see no reason the archetypes in those formats won't stick around, so if you ever think you might play Bant Eldrazi, Eldrazi Tron, BW Eldrazi, or Eldrazi Stompy, might be worth getting these before they rotate out.
Kalitas, Traitor of Ghet- Kalitas has held value ever since his printing, even as he's been largely relegated to sideboards in standard and the decks he was played in in modern fell out of favor for DS variants. Still a great card, though, and I don't think he's likely to go down in time, so I'd definitely make sure you have 1 or 2 in your collection if you play a decent amount of modern.
Kozilek's Return- Probably the less popular of the two modern 3 mana sweepers right now due to the prevalence of recurring creatures, but it's instant speed nature and ability to hit Etched Champion will keep it relevant. I think 1 of these should suffice for now.
SoI:
Thalia's Lieutenant- Fringe playable in the 4c/5c modern humans decks running around. I'm not yet convinced the deck will have staying power, but at least it's a fairly well supported "tribe", even if only inadvertently so.
Sin Prodder- Sees play in the new Mono Red Stompy decks in Legacy, but that's it. Not convinced it will remain a core part of the list longterm either.
Tireless Tracker- Not the all star in modern he was in standard, but it's still a super grindy CA machine, so I assume it will continue to see play here and there.
Traverse the Ulvenwald- One of the best cards on this list. Will continue to see play as long as they don't give it the GSZ ban in modern. Still played in abzan variants and Jund DS currently.
Nahiri, the Harbinger- Core part of one variant of Jeskai control in modern and one of the common choices for the pw suite in RW prison.
Prized Amalgam- 4x staple in Dredge.
EDM:
Selfless Spirit,
Mausoleum Wanderer,
Spell Queller- Along with the recently printed Collected Company, these made Spirits in modern a completely viable deck. Even outside that deck, Spell Queller sees play in some control lists and selfless spirit is an occasional one of include in chord/coco decks that want wrath insurance. Pretty sure Wanderer is a spirits exclusive, however.
Thalia, Heretic Cathar- Sees occasional play in Legacy DNT as a 1/2 of. Also played more frequently in modern hatebears.
Collective Brutality- Already valuable and for good reason; card is played in numerous modern and legacy sideboards.
Liliana, the Last Hope- What I just said about collective brutality x5. Hopefully you already have 1 or 2 here. If not, should probably snag them before they exit mass circulation.
Grim Flayer- Probably the best modern two drop printed since..... Tarmogoyf? Eidolon? Once again, these are already pretty pricey.
Man, Eldritch Moon was pretty crazy for eternal staples across archetypes as far as small sets go. I don't think it was opened nearly as much as the other sets on the rotation either iirc.
A lot of these rares are bargain bin already, thanks to the hunt for masterpieces and the sheer size of the print runs outside of them, so you may able to continue to get them at reasonable prices well past their standard tenure. It's also worth noting I didn't cover uncommons, so if duskwatch recruiter, cathartic reunion, blessed alliance, etc. ever become super prevalent, those could see a rise in value too. None of them seem like multi archetype all stars ala inquisition of kozilek though.
Let me know if I missed any others people should grab. I don't keep up too much with commander staples anymore and I don't know every constructed archetype in and out, so there are definitely blind spots in my review.
Maro is running a poll on Twitter "Who is the most famous Rick" if you are wondering what kind of things we will see in Unscrewed
Looks like the 2012 memes prediction might have actually been optimistic; we're going back to 2008. :/
