Slivers are one of the most prominent, aesthetically consistent and divisive creature types in Magic: The Gathering. There are just as many Sliver-lovers as self-proclaimed Sliver-haters, and with each new wave of support there's an influx of new Sliver decks waiting to dredge up past feelings of admiration or disgust. The Hivemind carries on, unthinking, unfeeling, ever-consuming.
Individual Slivers share a collective gimmick, with each one granting a specific ability or stat boost to your other Slivers. As your army grows, so too does the collection of keywords and mechanics that each one possesses. With over 100 Sliver creatures in existence, you'd think the well of abilities has run dry, but there are still plenty of tools at the Hivemind's disposal.
10 Storm
Hatchery Sliver opened the door for future Sliver-copying shenanigans, and there are plenty of Magic mechanics focused on cloning and copying creatures. Storm would add a similar new dimension to Sliver gameplay, encouraging correct sequencing and creative deckbuilding.
Storm counts the number of spells you casting, granting more copies of a spell the later in a storm chain you cast it. Lords like Predatory Sliver and Cleaving Sliver are already inherently good copy targets, let alone the fact that making three or four copies at once actually puts your opponents in lethal range. See Aeve, Progenitor Ooze – the first creature to have storm – for inspiration.
9 Annihilator
There's a reason annihilator has rarely been used post-Rise of the Eldrazi; it's miserable to play against and becomes problematic if it's too easily accessible. Still, there are certainly players who would jump at the opportunity to quite literally annihilate their opponents with these hypothetical Slivers.
The potential for annihilator Slivers was teased with the Mystery Booster playtest card Slivdrazi Monstrosity. While not tournament legal, playtest cards often serve as a lens into Magic's testing process for new cards and concepts. The prospects for a "black-bordered" Slivdrazi Monstrosity are narrow, but someone at R&D was thinking about it.
8 Legendary
Slivers have their fair share of 5-color legendary commanders, but what if all your Slivers could be legendary? Turning creatures into legendaries actually counteracts some token-making and copy effects, but it also opens up more opportunities for Slivers to combo with various "legendary matters" cards, of which there are plenty.
Transforming Slivers into the stuff of legends would make them highly compatible with Flowering of the White Tree, Heroes' Podium, and Bard Class, to name a few, and would give you access to potent one-sided board wipes like Urza's Ruinous Blast and Invasion of Fiora.
7 Delve
Cost reduction is universally welcome regardless of deck strategy or archetype, and Slivers could benefit immensely from converting cards in the graveyard into extra mana. Titans' Nest demonstrates how this would work, but simply putting delve on a sliver that then grants your other slivers delve would simplify the process.
There aren't actually that many high mana value Slivers, plus Gemhide Sliver and Manaweft Sliver already provide extra mana. Regardless, delve could help the viability of more expensive options like Constricting Sliver, Megantic Sliver and Fury Sliver.
6 Extort
Extort is closely associated with the Ravnica's Orzhov guild. It's a flavor-forward ability that might seem strange on an animalistic group of creatures, but it's a perfect fit from a purely mechanical perspective. Besides, who's to say Slivers can't get into racketeering and black market business ventures?
Individual instances of extort across your permanents add up and allow you to pay more mana to drain more life whenever you cast a spell. Pontiff of Blight is the prototype for what this looks like, and a Sliver-specific version would surely cost less than six mana.
5 Ninjitsu
Ninjas and Slivers: a match made in heaven, or an absolute nightmare? You decide. Slivers aren't known for nuance and trickery, but ninjutsu could add a strategic wrinkle that they're otherwise lacking. Various flash- and haste-granting Slivers can catch opponents off guard, but a ninjutsu sliver would do it in an entirely unexpected way.
A template like Satoru Umezawa's would lead to interesting mid-combat decisions, especially considering how many different abilities Slivers come equipped with. Imagine your opponent sneaking in Striking Sliver or Bonescythe Sliver with ninjutsu and completely changing the landscape of combat.
4 Prowess
It's surprising Slivers haven't received prowess yet. It makes some sense considering they're a creature-focused strategy and prowess benefits noncreature spells, but there's definitely room for the two to co-exist comfortably.
The Sliver Swarm Commander Masters precon included 19 noncreature spells, a healthy number of ways to trigger prowess without even trying. You could skew towards a spell-heavy build, using a prowess sliver as a go-wide board pump. It would take some restructuring of the typical Sliver formula, but at the very least it would be another tool in the Sliver arsenal that could lead to interesting deck construction.
3 Myriad
Myriad is the essence of "more," and Slivers are all about growth and propagation. They have natural synergy with any mechanic that allows them to multiply, though how well it scales depends on which Slivers you're attacking with.
Legion Loyalty sets a blueprint for what this effect might look like, but stapling it to a specific creature type probably results in a creature that costs less than eight mana. As great as myriad is in multiplayer games, it has no bearing in regular Constructed formats, which limits its chances of being used on future Sliver designs.
2 Venture Into The Dungeon
Venture Into the Dungeon is a mechanic that gets better en masse, and Slivers are well-known for their swarming capabilities. It's not hard to imagine a design that includes the text: "When a Sliver enters the battlefield, venture into the dungeon." That would allow Sliver decks to turbo-dungeoneer, and would give them an incentive to run initiative cards as well.
All existing dungeons are Dungeons & Dragons themed, so it might require some suspension of imagination to tie the two together. Perhaps this theoretical "venture Sliver" could release alongside a new Sliver-themed dungeon.
1 Decayed
For every Sliver-lover there's at least one other player who absolutely loathes them. Enter the "decayed" Sliver, one that makes them unable to block and limits them to a single attack. There's precedent for this sort of effect with Dormant Sliver and Shadow Sliver, which can add adverse abilities to opposing Slivers.
It's obviously a hyper-specific hoser against a narrow set of decks, and Slivers that grant abilities across the entire battlefield are a ten on Mark Rosewater's "Storm Scale." That means symmetrical Sliver abilities aren't even a consideration anymore.