Custom Card Game Design
If nothing else can get you to work together, maybe the end of the world can. In Siege of Ravnica, the guilds don’t get their own mechanic each. Instead, the guilds get a shared mechanic called Guildpact and each of the guilds will have a minor theme that can be built around. Guildpact is an ability word that counts the number of colors among permanents you control.
In many multicolor formats, you can try to draft five-color, but the results may vary. In Siege of Ravnica, you are directly incentivized to do so. You pick the best card from each pack, pick up the Guildpact cards when you can, but you are also smart and pick mana fixing over them! The Guildpact cards aren’t as good in two- or three-color decks, so you can pick them up later.
There are also some cards in the set that synergize well with Guildpact. For example, the “multi-guild hybrid” cycle. These creatures have a cost similar to the “Shardblade” cycle from Alara Reborn and can increase your color count drastically.
In addition to the synergy they provide with Guildpact, the hybrid cards tie together the multicolor environment of the set. Each of the two- or three-color combinations should be draftable and having flexible cards goes a long way in supporting that goal. The “guilds working together” theme could be expanded even more with true tri-colored cards at rare that have the watermarks of all three included guilds, but giving these cards a flavor that represents all three guilds reasonably well might prove to be too difficult.