Gaining control Permanently, without any limiter (such as Control Magic, you just have to destroy the enchant and you're good). Creatures, not fine. Enchants are fine. Artifacts are not-so-fine. Planeswalkers are fine. Lands not fine.

Discarding your opponents

Destroying one person's lands. If you make everyone sac lands, that's not as bad.

Accelerating everyone's lands, ie Collective Voyage. This is because a deck that works better with this, such as Jodah, becomes quickly OP. It would be nice if this mechanic was usable, a flourishing for all

All players drawing. Minds Aglow. This is because card economy is a whole metagame, with each deck finely balancing it's card expenditure versus card value, so when everyone just draws tons of cards, those decks that run lots of cheap things become broken... So I guess this is actually fine for a deck like Ral, cuz he'll use it as a wincon, but at the same time, people simply won't pay mana, but will still draw the cards, so ral might as well just draw himself the cards. So it's a lame, unusable mechanic in magic.

Unblockable, or Trample + any OP creature, such as infinitely large, or 10+ infect, etc. If a creature can't be blocked, and one-shots, it's just not fun, because there are very few ways to block an unblockable creature (Curtain of Light, and all those cards are trash, unusable in any synergy deck, only good in draft)... so you have to run Unsummon or Assassin's Trophy or whatever, which is the type of power creep that led to the spot removal and hard anti-creature meta. The solution is to make creatures always blockable... always make combat a thing. Even trample is OP, because creatures can easily get too much power, such as a doublestriker + Berserk. (Lesson: Don't play the big stompy game. It's unfun. Play good creatures, but don't make that your deck's theme / combo. Instead make your deck more defensive, more built around what you predict your opponents' decks to contain. Tailored meta. Because interaction is fun, and it isn't as well-engineered into MTG, it's clearly not a high focus concept, but it can be finagled if you know what you're doing. It's why shawn's simple decks are fun)

"Too OP" abilities? There are only Too Op abilities if they cause a game that has no interaction, causes others to be bored. Such as a 5-color goodstuff infinite combo deck. It is simply too strong for a lower tier meta. But it isn't actually too OP, because it's fun at a higher meta. Too OP is something that's NEVER fun. and I actually can't think of any examples in MTG, aside from the basic thought that 4-man ffa CEDH is lame. I don't think the decks themselves are lame, because under the right conditions the interactions and gameplay are highly entertaining. I just don't think 4-man free for all is the way to play CEDH. The same is probably true with Oathbreaker. 1v1's has been fun.

Teams? The teams meta definitely wants certain things: Slow, interactive gameplay between teammates, showing a unique thing in magic: Teamwork. This requires cards that target your teammate. It's especially good when those cards are simply part of the deck's mechanics, but they happen to mesh really well together, such as baby and slivers. But, since Magic doesn't think about team play enough, you really have to seek out these cards, and I believe there's an until-now-locked team meta to be unlocked by exploring combos between team-functional card mechanics. What does teams want to be(to be fun)? Basically it wants to be sorta like Draft teams, except featuring that special stack-abusing endgame of morphs, madness, ral, etc. But when those decks have mechanics that break teams, such as Ral's splash damage, it gets messy, necessitates removal spells. It wants creatures to be effective and threatening, ie "Combat matters". It still disdains beatdown aggro, such as Overwhelming Stampede + Berserk. It wants a good 10 turns before the game ends. That way, you all get a chance to see team mechanics intermingling. With this goal in mind, certain decks emerge as ideal teams decks. Decks that complement the weaknesses of each other, such as an Edric deck helping out a deck that's starved of card draw. But we also don't want decks that have the capability of getting out of control when another deck adds its' synergy. This would be double slivers, double elves, etc. Though as long as what they're doing reaches that "next level interaction", the game will still be fun. More research is needed with this "Casual-Competitive" level teams play. Each deck should definitely have a plan for this level 2 play, tho. Instant-speed responses. But that's also just a general advice for all decks. Which is why the whole spot removal meta formed, to combat broken combos with stunting abilities until you yourself can go off. So let's try for a bunch of decks that work this way. For oathbreakers, this most likely means the breakers should be somewhat slower, and designed to interact with eachother's meta. (Nahiri vs Ral, for example, or Saheeli using Dack to steal from Tezzeret, but then him playing Memnarch. Tezzeret's lifeloss abilities should be mitigated in teams, probably.)