Anti-tribal stuff is pretty fun, if a little impractical. I do think I would re-word it to be "number of creatures your opponents control that share a creature type," rather than "each," if that helps.

Leyline of Suppression

Enchantment

If Leyline of Suppression is in your opening hand, you may begin the game with it on the battlefield.

When Leyline of Suppression enters the battlefield, choose a creature type. (This still takes effect if you begin the game with it on the battlefield.)

Other permanents lose abilities that mention the chosen creature type.

Make another leyline card. Or make a commander focused around leylines.

The Most Powerful, Truest, Ultimate Counterspell

Instant

This spell can't be countered.

Counter all spells or abilities your opponents control on the stack.

"That's a real nice infinite combo you got there... It'd be a shame..."

I think this would count. Whirlwind Denial is legal (and what I used as reference to word it well).

Repeat with more legal un-set cards, this is fun.

Grotesque Vestige

Creature - Insect Spider Mutant

Flying, Reach

When Grotesque Vestige enters the battlefield, each player sacrifices a creature. For each generic keyword among those creatures, put a counter of that kind on Grotesque Vestige.

"It does nothing but adapt further and further, not thinking how practical all those extra body parts really are." - Zagoth Bonder

2/2

Let's see a four-color Dragon. Extra points if it's anti-red.

Caelia, Heroic Explorer

Legendary Creature - Faerie Elf Ranger

Vigilance

Whenever you cast an Adventure spell or a spell with an Adventure, create a 1/1 green Ranger token and scry 1.

Rangers you control get +1/1 for each Adventure card in your graveyard and/or exile.

3/5

Chosen for the Quest - Adventure

Search your library for an Adventure card, put it into your hand, then shuffle your library.

Love Bant adventures, have had the idea for building a Gorion, Wise Mentor for a little while. But yeah, had a lot of fun making this.

Make a legendary creature with a mechanic/keyword that hasn't been seen for a while/was only for one set.

Thantis, Sower of Aggravation

Legendary Creature - Spider

Vigilance, Menace, Deathtouch

Thantis, Sower of Hate can block as though it was two creatures.

At the end of each combat phase, for each creature the player whose turn it is controls, put a +1/+1 counter on it if it attacked this turn. If not, put a -1/-1 counter on it instead.

Whenever a creature dies, if it had less than one toughness, or was blocked by Thantis this turn, put a +1/+1 counter on Thantis.

3/6

A lot of text that's a little confusing, but I think it turned out well. It still keeps that idea of motivating players to attack, but not you because Thantis is very scary.

I decided to not outright force creatures to attack, because it isn't supposed to be as chaotic as its red version. I also made it more group-hug/pillowfort-y, as well as leaning into the counters because I felt that it was fitting of White and Abzan.

I really enjoyed this, and I want to see more. Make another color-shifted Thantis, preferably three colors.

Fudge Feaster

Artifact Creature - Ooze Food

Menace

, Sacrifice a Food: Destroy target creature with power less than Fudge Feaster's power. Put a -1/-1 counter on Fudge Feaster.

Eat or be Eaten

4/4

How about a red Ooze? Just to round out the color pie.

Not at all a science person tbh, but I will try.

Plasma Weird

Creature - Weird

Haste

: Plasma weird phases out. At the start of your next combat damage step, it phases in tapped and attacking.

"It worked! I finally figured out how to enhance its speed!... Where did it go?..." - Izzet Guildmage

3/2

Wild(s of Eldraine) (No, actually, do something that would fit in that set.)

Shatter the Veil

Enchantment

When Shatter the Veil enters the battlefield, create a Shard token.

The first time you activate the ability of a Shard token each turn, that ability cost to activate.

Love Niko, surprised that WotC hasn't done more with Shard tokens. Anyway I tried.

Now this is a bit broad, but make a card with a (non-combat related) keyword that breaks the color pie. Black with Miracle, or Red with Evolve, stuff like that.

Forgot to write "Spree" in there oops...

Lockup Beatdown +

Sorcery

• - Put a stun counter on each creature target player controls.

• - Tap target creature.

• - Destroy target tapped creature.

Had fun with this one, I love Azorius Tapdown, and I think it's pretty usable too.

Make a food card!

It is??? Why isn't it marked red then??? Aaaaaa

Probably... The problem really is trying to take something out for it... Probably Fight as One if I had to, but that's still a hard choice.

Make a Boros burn creature.

Since we're on a Sultai trend here...

Undergrowth Confluence

Sorcery

Choose Three. You may choose the same mode more than once.

• Surveil 2, then draw a card.
• Destroy target nonland permanent.
• Target creature gets +2/+2 and gains trample and deathtouch until end of turn.

I really don't like Mutagenic Growth, but I know it is good... I just really don't like paying life, but it would work so well. I guess Gitaxian Probe would be good as well. I'll add them both to the maybe board anyway.

As for Monastery Mentor I'm not sure, honestly it's more of a problem of what to take out for it. I think a good plan would be to reduce the Leonin Lightscribe and Faerie Guidemother to 3 to squeeze in 2 copies. I'll test around to see if it works.

Thanks for the suggestions anyway!

Make an Insect Horror creature.

Path // Paradise

Search your library for two basic land cards. Put one into your hand and the other on the top of your library

Aftermath

You may play two additional lands this turn. Draw a card.

I imagine that Path would have art of a nomadic person walking through the vast desert, almost tiny in the frame, with nothing but sand for miles around them. While Paradise has that same person in a beautiful oasis, with glistening water and a temple in the distance.

Not sure why a Path to Paradise card doesn't already exist, but here it is now.

Wish I could make it Jeskai, but alright.

Noble Wingbeast

Creature - Dragon Sphinx

Flying, Menace, Vigilance

Whenever you cast or copy an instant or sorcery spell, if it is red, Noble Wingbeast deals 4 damage to any target. If it's blue, draw two cards.

6/6

This might be a bit much for how easy you can cast I/S spells, drawing 2 cards and dealing 4 each time but hey, this is why it's a custom.

Make a rainbow colored charm with 5 different modes

Tsukimi Oh nice! Kobold/Dragon tribals are really cool, and this would actually fit perfectly into a Prossh, Skyraider of Kher Keep deck!

TypicalTimmy No... I think I've heard of them tho... I just like saying forgor, it's a silly word.

Yeah, I've got it now.

Something weird must have happened with the user tag and it I guess...