Designed a few random cards (1 per color) that I'd love to play in commander one day, including cards for R/B/G that actually give them something to do against Thoracle lines in cEDH (as the only ways to stop a Thoracle trigger at the moment is either Counterspelling or angel's grace).
Creature- Viashino Shaman (R)
Whenever a card leave a library other than the first card drawn during each player’s draw step, Mindpyre shaman deals 1 damage to that card’s owner.
While this card would feel equally at home as an Izzet, Dimir, Rakdos, or plain black card (seeing as underworld dreams is black), I could imagine a card like this being printed as a slight color bend to let red have a counter against heavy card draw or self-mill strategies.
Parting Word cannot be countered by spells or abilities if it is targeting a spell with mana value 2 or less.
Counter target noncreature spell.
While this card would need to fight for space against free spells like FoN/FoW/FG/PoN along with plain old Couunterspell/Manadrain/Swansong, I feel that having access to uncounterable counterspells (at least in very small numbers) is a good pressure valve against combo decks (especially as so many of them try to load up on their own counterspells to force a combo through).
Any number of target opponents shuffle their graveyards and all exiled cards they own into their libraries.
Draw a card.
A simple card that does relatively little but that can randomly rob some decks of their win-cons out of nowhere.
Creature- Human Assassin
Whenever an opponent would win the game, if you have not lost the game, that opponent loses the game instead.
I think that the language here works? Just a little card that stops opponents from seeking alternate win-cons until they get some kill/bounce.
Creature- Human Cleric Knight
Players may not sacrifice nonland permanents or discard cards to pay costs.
Basically the last remaining hatebear that white decks would want. I assume that "nonland" is needed as stopping fetches in eternal formats would be a bit too far but let me know if I am mistaken.
Mindpyre Shaman is too powerful as written: It turns mill into direct damage and mill is too undercosted to also do damage in that that way. If it said "Whenever one or more cards..." , though that wouldn't interact as positively with thassa's oracle. Alternately, "Whenever a spell or ability an opponent control's causes a card to leave that player's library,..." is more in line with your goal, but much more narrow in application.
Parting Word seems just not as good as Counterspell for the same cost and Fallow Fields almost could be 1 mana and still be fine for how minor the effect is.
Cunning Opportunist B
Creature- Human Assassin
If an opponent would win the game, that opponent loses the game instead.
Note that this only works against Alt Wincon cards. Zero life, Ten poison, and not being able to draw cause that player to lose, which removes them and their permanents from the game and then the remaining player wins by being the only player remaining. Because of this, the opportunist wouldn't be in play to cause that player to lose instead of win, because its controller had already lost.
Sanctifier is probably fine, and maybe even if it can hit land abilities too. Death and Taxes already hoses mana bases, so the question is this better than any of the cards the deck already runs? Maybe put it at 1WW to be safe if you let it affect lands.