Greater Turnabout
Instant (R)
Tap all permanents you don't control and untap all permanents you control. Exile this spell.
Turnabout is fair play.

Lay Off -
Sorcery
As an additional cost to cast this spell, sacrifice any number of creatures.
Create two Treasure tokens for each creature sacrificed this way.


Less abusable than Ashnod’s Altar since if you use your mana to make more guys, you can’t keep sacrificing them for more mana. But it gives colored mana

Clockwork Memnite -
Artifact Creature - Construct (U)
~ enters the battlefield tapped with three +1/+1 counters on it.
At the beginning of your upkeep, remove a +1/+1 counter from ~.
0/0

Knockoff Tarmogoyf -
Creature - Lhurgoyf
~'s power is equal to the number of card types among permanents on the battlefield and its toughness is equal to that number plus 1.
*/*+1

Air Bud -
Creature - Dog
Reach
Whenever ~ is dealt damage, it deals that much damage to you.
2/1

Mortgaged Property
Land
When ~ enters the battlefield, exile a card from your hand face down.
: Add or .
: Return the exiled card to its owner’s hand.

I am still shocked there isn't just:

Wild Griffin
Creature - Griffin (C)
Flying
4/2

Quicksilver Fisher is close

Can we push it further? We have Lightning Elemental and Conifer Strider


Tercio -
Creature - Human Soldier (C)
First strike
4/1


Peregrine Falcon -
Creature - Bird (C)
Flying
4/1


Brain Eater -
Creature - Zombie (C)
Lifelink
4/1

Lateland
Land
: Add .
: Add or . Activate only if you control three or more lands.


Makes colors one turn later than Graven Cairns, but maybe less risky in 3+ color decks

Think Fast -
Sorcery
Draw a card.
Storm


Die Hard with a Vengeance -
Sorcery
Return target creature card with mana value 2 or less from your graveyard to the battlefield with four additional +1/+1 counters on it.


Bulldoze -
Sorcery
Target creature gets +6/+6 until end of turn.
Return up to two target creatures to their owners’ hands.


Approval Poll -
Sorcery
Flip a coin for each creature you control. For each flip you win, draw a card.

Trash -
Sorcery
Convoke
Destroy target land.

Sealed Envelope —
Artifact — Clue (C)
, Sacrifice Sealed Envelope: Draw a card.
“To whoever may read this letter: It is already too late.”

I think part of the appeal of the original sealed envelope was that you could pay the cost in advance, so that later if you stacked the top of your deck you could draw the card right away for free. Or if you had effects that cared how many cards you drew in one turn

I came up with a way to count how many times your creature attacked the same player


Waltzing Matilda -
Legendary Creature
Whenever ~ attacks, defending player gets a poison counter. Then if that player has 3, 6, or 9 poison counters, ~ can’t be blocked this combat.
3/3

1-mana cycling is always pretty decent; they'd probably see a limited amount of play in monocolor EDH decks seeing as some still run the tapped cycling lands

As far as the original sealed envelope design goes, which as I recall was a U-cost artifact with free sac to draw a card, I personally like a bit more "Auspicious Augury" version of that design posted on reddit. I specifically remembered that since it was one of those designs I included in a custom set.

Just saw Invasion of Kaldheim for the first time, how it turns your hand into temporary cards instead of discarding. Interesting effect

Hyperdog -
Creature - Dog
Haste
When ~ enters the battlefield, sacrifice it unless you exile two cards from your hand. Until end of turn, you may play cards exiled this way.
4/4

The Goat -
Legendary Creature - Goat
: ~ gets +X/+Y until end of turn, where X is the greatest toughness among creatures your opponents control and Y is the greatest power among creatures your opponents control. Activate only once each turn.
0/1

Can you put energy symbols in mana costs? The mana value is still 2


Enerdrazi -
Snow Creature - Eldrazi
Whenever ~ attacks, it deals 2 damage to defending player.
It was previously believed that you couldn’t be struck by lightning and an avalanche at the same time.
3/3