Ok, with the release of Murders at Karlov Manor we need to re-evaluate the rules.

The idea behind the cube is to get all the cards that are representitive of ravnica. This means the sets that are just about Ravnica and nothing else are included in their entirety:

Ravnica City of Guilds
Guildpact
Dissension

Return to Ravnica
Gatecrash
Dragon's Maze

Guilds of Ravnica
Ravnica Allegiance

Some sets are placed on Ravnica but only partially about Ravnica. These sets I'll try and comb through and exclude only the cards that are not about Ravnica:

War of the Spark
Murders at Karlov Manor

Additionally I've also included one-off cards from core sets and supplemental products that are set on ravnica.