There's a cube in my area that many of my friends down at the local LGS all enjoy drafting. It's powered, I'm guessing around 600 cards, and it's loads of fun. But we've been drafting it for over a year (possibly more, but that's the length of time I've been going to that store), and after a while players of reasonable skill start to "solve" the set that they're drafting limited. When I decided to build my cube, I wanted to push off that date as long as possible.
When I design a split card, my goal was for it to pretty much fall into one of two categories:
- Both halves are of comparable power level, so that one half wasn't the one that got drafted for/played 100% of the time. So I'd avoid the "Sol Ring vs.Phyrexian Colossus" types of non-decisions.
- Each half of the card have a synergy with potential interactions with the rest of the board and with the other half of the card. (Something like Lightning Bolt paired with Chandra's Phoenix. You cast the bolt, then use the Phoenix half when it's in the graveyard to bring it back to your hand.)
- And after some preliminary set design, I decided on a potential third guideline: when possible, each half of the card be good to play at different stages of the game.