Kind of like mtg block constructed formats. Some sets are legal and others aren't. It's generally best if spells from the core set are always legal.
Arena Blocks
-Core, Forcemaster vs Warlord, and Forged in Fire
-Core, Paladin vs Siren, and Conquest of Kumanjaro
-Core, Druid vs Necromancer, and Conquest of Kumanjaro
-Core, academy core, academy priestess, and academy warlock
These blocks are intended to be optimized for balance and diversity, or at least diversity. As more sets get released, there will be more block formats added to this list. This list could also function as a useful guide to the order to buy different sets in for your playgroup. Sets that are in the same "block" will likely go well together and make for a fun, fairly healthy semi-casual metagame. Although if you're planning to make a competitive tournament with it, you might want to watch out for imbalances that only become apparent when taken advantage of by sufficiently skilled players.