Playing a lot with newcomers this last months and aiming to show them an accurate depiction of MtG's color identities, I made 10 bicolor 40 cards decks of each color pairs with clear mechanical theme:
Blink, Control/Appropriation, Agression/Suicide/Reanimation/Rudeness (!), Landfall, Token/Populate, Aristocrat, -1/-1 counters/Graveyard, Counters Proliferation, Spellslinger, Agression/Decisive Strike. Aiming to play at least 10 cards of my multicolor section in each deck, It happens that some cards wheren't able to make the cut for the wrong reasons: mainly being non-synergistic with what the colorpair does. Not every 2-color cards should be good together as they may be synergistic in 3-color deck. One exemple is Huntmaster of the Fells wich doesn't bring anything to a Landfall deck but should be great in a token/anthem/aggressive deck. Therefore, here is the first batch of card swaps that I made while testing my cube as a gauntlet of 2-color decks.

Ancient Grudge is too narrow for draft purposes. But this card cannot be replaced easily as its slot require a red card with an optionnal green cost, wich doesn't let me lots of options. Other pertinent options to replace it where Primitive Justice or Tin Street Hooligan but it happens that the cube is in more dire need of manaramp and manafixing than anti-artifact spells. Hence, Sprouting Goblin should do the job !

Impromptu Raid isn't worth it, Impromptu Raid isn't worth it, Impromptu Raid really isn't worth it. I though for a long time this card was an hybrid Sneak Attack but it isn't: the cost is too steep and even with a deck containing 22 creatures and 17 lands (plus that fatefull card), you'll alway have around 42% chances to fumble and lose your mana. Maybe if the activation cost was only one mana the card could work but what an horrible deck you would have to build to take advantage of that one card. On the other hand, Giantbaiting contribute to the Collective and Token Mechanical themes without hurdle. However, this last card could be way better and not that overpowered if only the giants wouldn't disapear at the end of turn (I'll probably make a custom version of that card soon).

Prosperous Partnership may just be a stricly better Goblin Trenches: it creates it's tokens rightaway and makes mana instead of sacrificing lands. Sure, it cannot be use to make a sudden and suicidal strike force of goblins but that last scenario happens so seldomly that I don't remember if it happens at least once !

Figure of Destiny is a deceptive design: yes it can be played turn one in lot of decks but only pure RW decks can get it to the Avatar level. More so, in an environment with levelers, it feel strange to not being able to spend mana more gradually to level it up. Boros Reckoner may also be the temporary choice for the RW hybrid section as it can be hard to play outside of a pure RW deck, but it's a solid creature that doesn't pose many more problems and may open the door to some combos. Spitemare could be a more playable alternative but it is way less agressive nor attractive.

Winding Constrictor is an anti-combo when using -1/-1 counters wich makes it awkward to draft most of the times. Making it go liberate more room for other mechanics and make the counter ramping/proliferating/doubling strategy an exclusively GU mechanical theme wich makes mechanical themes clearer overall.

Necropolis Fiend being replaced was predicted a long time ago but I never found a big black creature that could replace the top of my black section's curve. Including another -1/-1 counter themed card with Obelisk Spider was the occasion to rediscover Carnifex Demon when I rummage through old reddit posts about this average but interesting spider (maybe Obelisk Spider should be a candidate to a slight redesign to make it more potent and/or synergistic).

Slimefoot, the Stowaway may work by itself if you get enough mana to generate saprolings quickly. But played on turn three, it isn't a great card by any metrics. It was included because I loved it's character design. But, like we humans do sometimes, I didn't nurtured it enough to make it thrive. on the other way, Glissa Sunslayer feels like a sturdy creature wich check lot of boxes.