New minions are unlocked by scoring points with that clan, so if you play with minions from a clan you'll unlock more of them for the next game. With the full game release coming in less than a month, I've limited the demo to include just what can be unlocked in 2-3 normal playthroughs, so there are some minions (especially in the Corrupted & Enchanted clans) who will only be unlockable in the full game (as well as the majority of relics).
I'd appreciate any feedback about the variety available here actually, as I do want the demo to offer the full game experience while still of course giving people something extra if they decide to purchase. If you think the options are too limited here I'd consider increasing what's available.