One of the most insightful quotes I heard about retail Limited was that 'every deck is midrange' - the card quality is low enough, and most cards generic enough, that most decks land (possibly uncomfortably) in the midrange zone of whatever classification you use. Most Limited formats have a dedicated aggro deck - sometimes it's explicitly supported in a colour pair or role (Mono Red in Shadowmoor), sometimes it's an emergent property of the format that gets discovered later (non-Shard BR in Shards of Alara), sometimes it's not actually there but you can pretend it is if you get scrappy. Most other decks in most of these formats are midrange - maybe they have an identity (this one cares about artifacts or graveyards or...) but they are an artifact midrange or graveyard midrange deck.
Most Limited decks will have a broad mana curve - ideally a lot of 2 drops or 3 drops going up to a few 5s or 6s. The aggro decks aren't too fast to end the game before Villain casts their expensive cards and you're likely to be able to cast some more expensive curve-toppers yourself. This changes as the format's power level increases - aggro has a much sleeker, narrower curve and has to look more distinct from midrange. At its most extreme, you end up with most Vintage Cubes containing a dozen variants on Dragon Hunter or Jackal Pup because their aggro decks need such a high density of one-drops to function (this is highly damaging to those formats in many ways IMO but that's for another thread). Dedicated control decks are also more likely in a custom, high-powered format, especially as the enfranchised players who tend to own Cubes have usually played for a long time and have some affection for the era of Magic where pure control was popular. However, these are more likely to resemble the tap-out control decks that are common in Constructed today, with counters acting as a bridge to your finisher - Keiga in 2005, Dream Trawler today.
What this often means is that the aggro decks want a lot of very specific cards and some generically strong cards, which are poached by midrange decks; the control decks mostly resemble midrange decks outside of a few cards; and midrange is just a label we stick on to whatever's left. In my ideal world, we move back towards that original claim about Limited but from a new angle - the aggro decks are capable of playing as midrange decks and the midrange decks can have aggressive starts. Like the red decks and Mardu Vehicles from recent Standard, I also want my aggro decks to be able to pivot to midrange in sideboarding - and vice versa to some extent for midrange decks (like the Abzan Control decks that would SB in Fleecemane Lion). For example, my white aggro deck can apply early pressure with Student of Warfare, Figure of Destiny or Kytheon but these cards have enduring relevance at all phases of the game; in the midgame I can use Flickerwisp or Restoration Angel shenanigans to flood the board with permanents. I go more into this dynamic
here.
Softening the boundary between different deck types makes their identity less clear but also ensures that midrange decks and the gameplay they encourage occurs in a higher quantity of games. It also allows you to dedicate space for classic midrange - The Rock straight out of the early 2000s - without worrying about parasitism there.
One of my favourite midrange cards of all time is the elephant in the room:
When I started playing Magic, this was a preview that stunned people; Siege Rhino was still a decade away. The hardest working pachyderm in Ravnica was a bane of aggro decks and a boon for aggro decks -
Congregation at Dawn for three Hierarchs was a common tactic in Block Constructed and five-set Standard. If you were playing some generic WGx deck and your opponent led on Urza's Tower or basic lands for Heartbeat Combo, you could at least hope to land an early elephant and get to attack a few times.
In this era of Magic, the consensus was that the matchup cycle worked like this:
Aggro > Control > Combo > Aggro
or
Aggro > Control > Midrange > Aggro
The midrange decks back then were comically bad against control - your threats were easy to remove or counter either at a profit, a mana advantage, or both, and control would eventually get to play threats that matched or outclassed yours.
Planeswalkers helped to change that, providing threats that generated a cascading advantage, were immune to most sweepers, and couldn't be easily contained in combat by control. In that vein:
The OG Garruk is still the perfect planeswalker design in my mind. It can threaten to win the game if you have a board presence, build one if you don't, or ramp you towards the powerful 5-6 drops or X spells that give you your endgame power. The ability to +1 and follow up with a 2-drop is very strong and encourages you to draft/build with your curve in mind.
These card advantage creatures in green have been the bedrock of midrange in Standard for the past 5+ years and you don't need many games with them to see why. None of them are overpowering but all of them are exactly what you want to see in the early tuns (or the late ones!) with a midrange deck. I love that these are bread-and-butter cards that also encourage more adventurous impulses - a Lands deck in green can happily use all of these, a Blink deck loves Jadelight and Nissa etc
Monarch is a controversial mechanic but Jailer is a great tool for midrange decks that can defend your crown in combat but also capitalize on the card advantage better than aggro
Very wordy but I've loved the subgames this creates and it leads to some incredible comebacks and stories