Not necessarily very good ones, but ideas, still.

Five-Color Enchantments

Again, going back to the Theros Block decks, there was a pretty strong enchantment deck built around card advantage from [scryfall]Eidolon of Blossoms[/scryfall] and [scryfall]Courser of Kruphix[/scryfall] while taking over the game with [scryfall]Doomwake Giant[/scryfall].

Well, the difference is that mana is now much better with the wedge lands. Not that much better, but if the format is slower then the previous standard, this might be a good idea. Just slow your opponent down enough to use [scryfall]Extinguish All Hope[/scryfall]. Sure, it’ll kill your [scryfall]Sylvan Caryatid[/scryfall]s, but that’s something you can live with if the rest of your board stays in tact, while your opponent loses his. Sure, there will be some enchantment creatures on the opposing side, so this deck is (once again) very metagame dependent.

[deck]
[creatures]
4 Courser of Kruphix
4 Eidolon of Blossoms
4 Doomwake Giant
4 Brain Maggot
3 Chromanticore
1 Keranos, God of Storms
1 Thassa, God of the Sea
1 Ephara, God of the Polis
2 Pharika, God of Affliction
4 Sylvan Caryatid
[/creatures]
[spells]
4 Banishing Light
4 Extinguish All Hope
[/spells]
[lands]
4 Mana Confluence
4 Temple of Malady
4 Jungle Hollow
2 Llanowar Wastes
2 Opulent Palace
2 Sandsteppe Citadel
3 Temple of Enlightenment
3 Temple of Abandon
[/lands]
[/deck]

BW Warriors

Supposing either the monoblack disruptive aggro (or Suicide Black) or monored aggro or both are popular, this might just be big enough to beat those, but at the same time fast enough to go below all the three-colored goodstuff decks.

[deck]
[creatures]
4 Bloodsoaked Champion
4 Tormented Hero
4 Seeker of the Way
4 Mardu Skullhunter
4 Chief of the Scale
4 Chief of the Edge
3 Underworld Coinsmith
[/creatures]
[spells]
4 Murderous Cut
4 Despise
4 Thoughtseize
[/spells]
[lands]
4 Caves of Koilos
4 Temple of Silence
6 Plains
9 Swamp
[/lands]
[/deck]

Ascendancies in Modern

I don’t play much modern, so I wouldn’t ever have figured these out, but apparently [scryfall]Mardu Ascendancy[/scryfall] and [scryfall]Jeskai Ascendancy[/scryfall] can produce turn three kills. Actually, the Jeskai one can do it in two, if lucky.

With Mardu, the combo is turn one [scryfall]Birds of Paradise[/scryfall], turn two [scryfall]Doran, the Siege Tower[/scryfall], turn three [scryfall]Mardu Ascendancy[/scryfall]. Attack with your two creatures and sacrifice the Ascendancy. You’ll do 8 (Doran) + 4 (Birds) + 4 (Goblin 1) + 4 (Goblin 2) = 20 damage.

With Jeskai, the combo is to use every cantrip imaginable, but the key is [scryfall]Gitaxian Probe[/scryfall], because its “free” (you don’t need mana to cast it). Again, first turn [scryfall]Birds of Paradise[/scryfall], second turn [scryfall]Jeskai Ascendancy[/scryfall], then cast the Probe, untapping the Birds and continue with your cantrips until you find another probe or just don’t want to go further and attack with the Birds.

I see bans in the future.