Finally getting this cube up to speed and creating a solid environment that I'm now happy to assemble and playtest in the future.
Notably black now features +1/+1 synergy, making it the fourth color.
Counters really brings the cube together and is a great catch-all synergy for most 3-color decks.
Blue seems the least tightly constructed color at the moment and didn't have many possible inclusions from sets in the last year. I was considering artifacts for blue but the overlapping synergy seems too fringe with sacrifice and counters.