First, let me say this: I don’t see this being implemented, but I do like the idea.
How about some of the banned cards are conditional? You can play them as long as you don’t play them in the same deck with another card.
Example: Felidar Guardian got the axe for being too good with Saheeli Rai, forming an efficient turn four infinite combo, where one part of the combo can be used to find the other and one part of the combo is just very efficient, making the deckbuilding cost of playing the combo very low.
The thing is, Saheeli Rai is a mythic face card of a set. It wasn’t going to get banned unless it was a problem in other ways (like Emrakul). So, the poor uncommon cat was pushed out. However, Felidar Guardian has a lot more uses in the format. For example, I wanted to play it with a bunch Cartouches and Trials.
That’s just one wild idea. There’s so many strong ETB abilities in the format that I’m pretty sure Felidar Guardian would have seen a lot more play than Saheeli Rai has since the banning (which is basically none).
Sure, it’s not quite that simple. You know certain segment of the player base will immediately ask about sideboarding, which is something that would need to be addressed in the rules.
However, more options is always more options. Felidar Guardian is a strong card with multiple uses. It is also an uncommon, letting players play something powerful without having to make a huge investment.
However, would it be worth it? It would add complications to the rules and not for much benefit. How many cards like this are there? The Standard banned list looks currently like this:
Attune with Aether
Besides the cat, all the other cards have been banned, because they are too powerful. They are not used as part of a two card combo, but instead they push certain decks over the top. (You could probably unban Aetherworks Marvel now that the Eldrazi are gone.) Trying to pair any of them into these conditional bans would be quite hard.
In other formats, you run the risk of looking foolish, when a new card comes along and replaces one of the pair. There’s also the problem of having too many options, although Pestermite is a lot more fragile than Deceiver Exarch, but that wouldn’t exactly make Splinter Twin safe (although it too, might actually be safe). Looking at Modern banned list, this approach wouldn’t do much. Maybe allow for Punishing Fire to be banned only conditionally, but would that even matter, as it probably wouldn’t do anything without this particular combo?